Quick Guide to Developing a Mobile Payments App

Developing an app with mobile payments is not just about integrating a gateway.

In fact, what it implies is the creation of a coherent flow that reduces friction and ensures that each transaction is correctly confirmed in the backend for user and merchant security. 

  • In the modern world, mobile is the main channel of digital interaction.

  • A poor checkout experience can trigger abandonment in seconds.

  • For that reason, if you're evaluating specific integrations or popular methods on the market, it's a good idea to first review what Errors when programming you can avoid before you start writing code. 

1. Define the architecture before touching the SDK

The most common mistake is to start with technical integration without defining the business model and special cases.

Keep in mind:

  • The architecture will not be the same,
  • that for a local perfume store that wants Receive card payments.

That's why there are questions you need to answer:

  • ¿It will be a one-time payment, Subscription o Recharge system?
  • ¿The charge is executed within the app or via redirection?
  • ¿How cancellations and refunds are handled?
  • ¿What happens if the user loses connection in the middle of the process?

Approach comparison table

In this case, the most important thing, as in any development, is that the foundations you define are coherent with the nature of the project.

Choosing well here avoids costly refactorings later on.

Approach Where the payment happens Level of control Regulatory Responsibility Ideal for
External checkout (redirect)) Provider page Low Very low MVP, Rapid Validation
In-app SDK Inside the app Medium Media Apps with a focus on UX
Direct API (server-to-server) Own backend High High Experienced and high-volume teams
External wallet (deep link) External app Medium Low/Medium Recurring e-commerce

Things to keep in mind:

  • Redirect reduces risk, but sacrifices UX (user experience)).
  • SDK facilitates development, but requires correct state management.
  • Direct API it implies greater responsibility for validations, security and compliance.
  • External wallet depends on well-managed deep links and solid fallback.

2. Design the UX flow first (and then the code)

While you may be eager to start writing the program, always keep in mind that UX comes first and that an efficient payment app usually has 3 key screens:

  1. Payment Summary: Amount, concept and possible commissions.
  2. Clear confirmation: A single action button.
  3. Result: success or mistake with a clear next step.

Best UX Practices in Payments

  • Minimizes manual fields.
  • Avoid ambiguous text such as "Unknown error”.
  • Always shows processing status.
  • Allows you to change method without restarting the entire flow.
  • Save the user's preferred method always asking for their consent.

Here you will realise that:

A simple and straightforward flow Reduces churn rate. 

3. Recommended technical step-by-step

Although there is more than one way to approach the development of a payment app, in general this pattern is the most stable in production, and that is why if you are just starting out we recommend you to follow it.

Flow Chart

Below is a brief diagram about the architecture that we are going to analyse:

Usuario
   ↓
App móvil
   ↓
Backend (fuente de verdad)
   ↓
Proveedor de pagos
   ↓
Webhook → Backend
   ↓
Base de datos actualizada
   ↓
Notificación a la app

This pattern is known as Payment Intent Pattern.

Most current providers work with a model based on payment intent:

  • The backend creates the intent.
  • A secure identifier is generated.
  • The customer confirms the payment.
  • The provider notifies the backend via webhook.

This pattern avoids relying on the user's device as the source of truth, For safety.

Let's take a closer look at the step by step.

Step 1: Create payment intent in the backend

The backend generates the order with:

  • amount
  • Currency
  • order_id
  • Metadata

The app does not have to calculate final amounts on its own, but in this case the server is the source of truth.

Step 2: Send the app a secure session

The backend will then return a secure identifier (client_secret or session_id) that the app will use to initiate the payment.

Step 3: Execute the payment

You can use:

  • Built-in SDK.
  • Redirect to secure page.
  • Deep link to external wallet.

Here it is important to manage:

  • Cancellations,
  • Interruptions,
  • Timeouts.

Step 4: Webhook confirmation

Never rely only on what the user's device says.

The backend must receive official confirmation from the provider via a webhook:

  • payment_succeeded
  • payment_failed
  • requires_action

Only then will you update the status in the database and notify the app.

Step 5: Implement idempotency

To avoid duplicate charges:

  • Generate a idempotency_key.
  • Reuse it if the user retries the same payment.

4. Deep links and intelligent fallback

However, if the payment involves opening another application, such as PayPal, Deep links allow you to start the process directly in that app with the data preloaded.

However, you should anticipate failure scenarios.

Fallback Table

Scenario Real technical risk Recommended strategy UX Action
App not installed Deep link fails silently Detect schema before invoking Show web alternative button
Blocked Redirect No reliable callback Polling or re-consultation by order_id Show status "Verifying payment"…”
User cancels Ambiguous status in client Confirm status via webhook Offer immediate retry
Network Timeout Order is in pending status Idempotency + revalidation Allow to continue without duplicating collection
External app does not return Interrupted flow Persist local state + sync when opening Resume Flow Automatically

A well-designed fallback prevents the user from perceiving the failure as insecurity.

5. Minimum essential security

Even if you don't store cards directly, you should still apply:

  • Tokenization.
  • HTTPS required.
  • Signature validation in webhooks.
  • Logs without sensitive data.
  • Handling suspicious retries.

In addition, he recalls Separate test and production environments.

Scope of Compliance

The scope of compliance, or level of regulatory responsibility, depends on how much control you have over sensitive data.

The more control you take on, the greater your security, audit, and regulatory responsibility.

For example:

If your system processes cards directly, you must comply with PCI DSS.

But don't worry:

Most modern applications delegate the handling of sensitive data to the vendor through tokenization, reducing the scope of audit and operational risk.

Anyway, I'll tell you a little more, in case it's of interest to you.

¿What is PCI DSS?

PCI Security Standards Council defines the PCI DSS standard.

The acronym stands for: Payment Card Industry Data Security Standard.

It is a set of mandatory safety requirements if:

  • You process cards directly.
  • You store card data.
  • You transmit sensitive data.

It includes requirements such as:

  • Strong encryption
  • Network segmentation
  • Access control
  • Auditable logs
  • Vulnerability scanning

6. Metrics you should measure from day one

Without data, there is no real optimisation.

Monitor:

  • Checkout conversion rate.
  • Average Completion Time.
  • Abandonment by screen.
  • Most frequent types of errors.
  • Retry rate.
  • Successful Payouts vs Total Attempts.

These metrics will allow you to detect technical or design friction.

Conclusion

The best way to start developing a mobile payment app is:

  • Build a solid architecture and clear flow first,
  • and then integrate the chosen technology.

A robust system is not the most complex, but the one that:

  • Confirm transactions securely,
  • reduces unnecessary steps, and
  • Allows iterating with real data.

A well-designed payment system goes beyond Happy Path.

  • It involves testing: reconnects, retries, duplicate webhooks, and intermediate states.
  • This really demonstrates the robustness of the implemented solution.