Expert Guide Series

Can I Build a Secure Payment System on My Own?

More than half of all apps that collect payments will experience some form of payment-related issue in their first six months, from failed transactions to security vulnerabilities that could have been avoided. The question of whether you can build a secure payment system on your own is one I get asked probably once a week, and the answer is both simpler and more complicated than most people expect.

Building payments into your app is one of those things that looks straightforward until you actually start doing it, then you realise there are about fifty different considerations you hadn't thought about. I've worked with solo developers who've spent months building custom payment solutions only to scrap everything when they discovered what compliance actually requires, and I've also seen independent developers successfully integrate payments in a weekend using the right tools.

The gap between these two outcomes comes down to understanding what you're actually trying to achieve. You don't need to become a payment security expert to accept money in your app, but you do need to know which parts of the payment process you should handle yourself and which parts you should absolutely leave to someone else.

Most payment-related app failures happen not because of technical skill but because developers don't understand the difference between processing payments and handling payment data

Understanding What Payment Integration Really Means

When people talk about building a payment system, they're usually talking about three completely different things without realising it. There's the user interface where people enter their card details, there's the actual processing of those details to move money around, and there's the infrastructure that keeps everything secure and compliant with banking regulations.

The confusion happens because good payment systems make all three look like one seamless experience. A customer taps their card details into your app, money appears in your account, everyone's happy. What they don't see is the massive complexity happening behind that simple interface.

The frontend collection of payment information from your users
The tokenisation process that turns sensitive card data into safe references
The actual communication with banks and card networks
The fraud detection systems running in the background
The reconciliation systems that match payments to orders
The refund and dispute handling processes

I've built payment integrations for fintech apps where getting these pieces wrong would have meant financial losses measured in thousands of pounds, and the thing that saves solo developers is that you don't have to build all these pieces yourself... you just need to know which ones you're responsible for.

The PCI DSS Compliance Reality Check

The Payment Card Industry Data Security Standard is a set of requirements that anyone handling card payments needs to follow, and this is where most solo developers hit their first major obstacle. The standard exists because card fraud costs the industry billions every year, and the card networks decided everyone involved in processing payments needs to meet certain security requirements.

There are four compliance levels based on your transaction volume, but even the lowest level requires you to complete a Self-Assessment Questionnaire and potentially undergo quarterly security scans. If you're storing, processing, or transmitting cardholder data yourself, you're looking at the full compliance requirements which include things like maintaining a firewall, encrypting data transmission, regularly updating systems, restricting access to cardholder data, and maintaining detailed security policies.

Compliance Level	Transaction Volume (Yearly)	Requirements
Level 1	Over 6 million	Annual audit by security assessor
Level 2	1-6 million	Self-assessment questionnaire
Level 3	20,000-1 million	Self-assessment questionnaire
Level 4	Under 20,000	Self-assessment questionnaire

The simplest way to reduce your compliance burden is to never let card data touch your servers at all. Using a payment provider's hosted checkout or client-side tokenisation means you can often complete the shortest version of the compliance questionnaire, which takes about an hour instead of several days.

Why Most Solo Developers Should Never Handle Card Data Directly

I've probably built thirty or forty payment integrations over the years, and I can count on one hand the number of times when handling card data directly was the right choice. The liability alone should be enough to put you off, because if there's a data breach and card numbers get stolen, you're potentially on the hook for the fraud costs.

The technical challenges are significant too. You need to encrypt data both in transit and at rest, you need to maintain detailed logs of who accessed what data and when, you need to keep all your systems patched and updated, and you need to segment your network so that if one part gets compromised the payment data stays safe. This connects to broader API security considerations that many developers overlook.

Set up and maintain PCI-compliant server infrastructure
Implement proper encryption for data storage and transmission
Build secure authentication and access control systems
Maintain detailed audit logs of all payment activities
Conduct regular security assessments and penetration testing
Create and maintain security policies and procedures

But here's the thing that really matters for solo developers... doing all this properly costs money and takes time away from building the features that make your app valuable. I worked on one project where the founder spent six months building a custom payment system because he thought it would give him more control, then switched to Stripe and implemented the same functionality in three days.

Payment Service Providers That Actually Work for Independent Developers

The modern payment provider landscape gives solo developers options that simply didn't exist when we started building apps. Stripe has become the default choice for good reason because their documentation is excellent and their API is genuinely pleasant to work with, but they're not the only game in town.

For UK developers, GoCardless works brilliantly if you're dealing with recurring payments and Direct Debit makes sense for your business model. The fees are lower than card payments (typically around 1% rather than the 1.4% plus 20p you'd pay for card transactions), but you're limited to bank-to-bank transfers which take a few days to clear.

The best payment provider for your app is the one that handles your specific transaction patterns well, not necessarily the one with the most features or the lowest headline rate

PayPal and Braintree (which PayPal owns) give you slightly different trade-offs. PayPal is recognised by nearly everyone which can increase conversion rates, but their dispute resolution process tends to favour buyers heavily. Braintree gives you a more developer-friendly API while still offering PayPal as a payment option, which can be the best of both worlds.

Square works well if you need to tie together physical and digital payments, and Adyen is worth considering once you're processing serious volume (we're talking six figures or more), because they can often negotiate better rates than you'd get with the standard providers.

Building Your First Stripe Integration the Right Way

I've watched developers make the same mistakes with Stripe integration enough times that there's a clear pattern to what works and what doesn't. The temptation is to jump straight into processing payments, but the right approach is to set up your webhook handling first before you process a single transaction.

Webhooks are how Stripe tells your app about things that happen asynchronously, like when a payment succeeds or fails, when a subscription renews, or when a customer disputes a charge. If you build your payment flow without proper webhook handling, you'll end up with customers who've paid but don't have access to what they bought, or subscription renewals that happen but don't extend the user's access.

Install the Stripe SDK for your platform (they support iOS, Android, and every major web framework)
Set up webhook endpoints in your backend before processing any payments
Implement idempotency in your webhook handlers to prevent duplicate processing
Use Stripe Checkout or Payment Element rather than building custom card forms
Store the Stripe customer ID and payment method ID in your database, never the card details
Handle payment failures gracefully with clear messaging to users

The Payment Element is Stripe's newer component that handles all the UI for collecting payment information, and it automatically supports whatever payment methods you've enabled in your Stripe account. This means you can add support for Apple Pay, Google Pay, or local payment methods without changing your app code.

Apple Pay and Google Pay Implementation for Solo Projects

Adding Apple Pay and Google Pay to your app does two things... it makes checkout faster for users who've already saved their cards to their device, and it increases conversion rates because people trust the device-level payment systems. The implementation is simpler than you might think if you're already using a provider like Stripe, because they handle most of the complexity for you.

For Apple Pay, you need an Apple Developer account (£99 per year), and you need to verify your domain with Apple if you're building a web app or create the right entitlements if you're building a native iOS app. The actual payment flow uses the same Stripe integration you've already built, it just collects the payment information through Apple's interface instead of a standard card form.

Payment Method	Setup Time	Conversion Lift
Standard Card Form	Baseline	Baseline
Apple Pay	2-3 hours	10-15% higher
Google Pay	1-2 hours	8-12% higher

Test your Apple Pay and Google Pay implementation on actual devices, not just simulators. The simulators work differently enough that you can have a working simulator integration that fails on real hardware, particularly around the biometric authentication flow.

Google Pay is slightly more straightforward because there's no annual developer fee and the verification process is simpler. You register your app or website with Google, add the Google Pay button to your checkout flow, and handle the returned payment token through your existing payment provider.

Testing Payment Flows Without Risking Real Money

Every payment provider gives you test credentials that work with special card numbers to simulate different scenarios. Stripe's test mode is particularly good because it lets you simulate not just successful payments but also specific failure cases like insufficient funds, expired cards, or cards that require 3D Secure authentication.

The test card number 4242 4242 4242 4242 will always succeed in Stripe test mode, while 4000 0000 0000 0002 will always fail with a card declined error. There are test numbers for basically every scenario you might encounter, from cards that require authentication to cards that will trigger fraud detection. This payment testing should be part of your broader feature testing strategy before rolling out to users.

Test the happy path where everything works perfectly
Test card decline scenarios and verify your error messages make sense
Test 3D Secure authentication flows on both desktop and mobile
Test webhook delivery by using the provider's webhook testing tools
Test refund processing from your admin interface
Test payment method updates for subscription scenarios

The thing that catches people out is testing the webhook flow properly, because webhooks arrive asynchronously and might be delivered multiple times if your server is slow to respond. Use the provider's webhook testing dashboard to replay events and make sure your code handles duplicates correctly.

When Your App Needs More Than Basic Payment Processing

The basic payment integration gets you surprisingly far... you can handle one-off purchases, subscriptions, and refunds with the standard features from most payment providers. But some apps need more complex payment handling, and it's worth knowing when you're crossing into territory that needs custom development.

Marketplace apps where you're facilitating payments between multiple parties need something called split payments or platform fees, where money moves from a buyer to a seller but you take a cut. Stripe Connect handles this well, but the integration is more complex because you're now managing accounts for your sellers as well as processing payments. For financial services like banking apps, features such as card controls and freeze functionality require additional development considerations.

The moment you're holding money on behalf of other people, you've moved from simple payment processing into financial services territory, and that comes with its own regulatory requirements

Subscription apps with multiple tiers, add-ons, and prorated upgrades need careful handling of the billing logic. Most payment providers give you the building blocks but you need to implement the business logic yourself, particularly around things like giving credit for unused time when someone upgrades mid-billing period. Understanding pricing psychology can help you design these upgrade flows to maximise conversions.

International apps selling to multiple countries need to handle currency conversion, local payment methods, and varying tax requirements. The payment provider can handle the currency conversion, but you need to think about how you display prices, whether you absorb the conversion fees or pass them to customers, and how you handle refunds when exchange rates have moved. For insurance companies looking to build trust through their apps, clear payment interfaces are particularly important for policy management and claims processing.

When building complex payment systems, developers often underestimate the security implications across different platforms. Cross-platform security considerations become critical when you're handling sensitive financial data across iOS and Android apps. Additionally, ensuring proper user consent management is essential for payment data collection and processing compliance.

As your payment processing grows more sophisticated, you might find yourself needing to consider whether to split payment functionality into separate products, especially if you're building marketplace or multi-tenant solutions where different user types need different payment experiences.

Conclusion

Building secure payment processing into your app as a solo developer is absolutely doable if you use the right tools and don't try to reinvent wheels that have already been perfected by payment providers. The key is understanding that you're building the user experience and business logic around payments, not the actual secure processing of card data.

Use a proper payment service provider, implement their client-side collection tools so card data never touches your servers, handle webhooks properly so you catch asynchronous payment events, and test thoroughly with the test credentials before going anywhere near real transactions. Do those things right and you can have a secure, compliant payment system running in your app within a few days rather than the months it would take to build everything from scratch.

If you're working on an app that needs payment integration and want someone with experience to either build it for you or review what you've already done, get in touch and we can talk through what makes sense for your specific situation.

Frequently Asked Questions

Using a service like Stripe, you can have basic payment processing working in a weekend if you follow their documentation properly. However, implementing proper webhook handling, testing all failure scenarios, and ensuring compliance typically takes 1-2 weeks for most solo developers.

PCI compliance levels are based on your annual transaction volume, ranging from Level 4 (under 20,000 transactions) to Level 1 (over 6 million). Most solo developers start at Level 4, which requires a self-assessment questionnaire that takes about an hour if you're using a payment provider's hosted solution.

You should never store actual card details in your database due to PCI compliance requirements and security risks. Instead, use your payment provider's tokenization system to store secure customer and payment method IDs that reference the card details stored safely on their servers.

Start with basic card payments using your provider's payment element, then add Apple Pay and Google Pay as they typically increase conversion rates by 10-15% and only take a few hours to implement once your core payment flow is working.

Payment webhooks can be delivered multiple times, so your code must handle duplicates by checking if you've already processed each event. Most providers will retry failed webhooks for several days, so implement proper error handling and use idempotency keys to prevent duplicate processing.

All major payment providers offer test modes with special card numbers that simulate different scenarios. For example, Stripe's test card 4242 4242 4242 4242 always succeeds, while 4000 0000 0000 0002 always fails with a decline error.

The biggest mistake is trying to handle the payment processing before setting up webhook handling properly. This leads to situations where customers pay successfully but don't get access to what they bought because the app doesn't receive the payment confirmation asynchronously.

Custom payment solutions only make sense for complex scenarios like marketplace apps with split payments, or when you're processing very high volumes and need specific cost optimizations. For 95% of apps, using providers like Stripe or similar services is faster, more secure, and more cost-effective.