Payment App Development

Most applications that add payment functionality are not building payments infrastructure; they are integrating it. The engineering challenge is not implementing a clearing engine but making PSP integrations reliable, keeping card data out of your infrastructure to minimise PCI-DSS scope, designing payment UX that converts rather than abandons at checkout, and handling the full lifecycle of a payment transaction including webhooks, retries, refunds, and disputes.

The scope of that challenge expands significantly when the application handles multiple payment methods (cards, bank transfers, digital wallets, local schemes), operates across multiple jurisdictions with different regulatory obligations such as SCA under PSD2, or manages recurring billing where failed payment recovery is a direct revenue impact.

Scrums.com builds payment-enabled applications for FinTechs, SaaS companies, marketplaces, and financial institutions. Our dedicated engineering teams deliver consumer payment apps, embedded payments in platform products, and merchant-facing payment tools built on top of leading PSP and open banking APIs.

Payment Application Architecture

Reliable payment applications have distinct concerns at each layer. Getting PSP integration patterns right from the outset determines PCI scope, checkout conversion, and the long-term maintainability of the payment flow.

PSP Integration Architecture

Payment Service Provider integrations (Stripe, Adyen, Braintree, Checkout.com) are the execution layer for most payment applications. The integration pattern matters: PSP-hosted checkout pages minimise PCI scope to SAQ A but limit UX customisation; PSP-provided JavaScript components such as Stripe Elements or Adyen Drop-In retain more control while keeping card data off the application server; direct API integration requires SAQ D compliance and is appropriate only when specific PSP limitations cannot otherwise be worked around. We select the integration pattern based on your PCI scope tolerance and UX requirements before implementation begins.

Webhook Handling and Payment State

PSP webhooks are the primary mechanism for receiving asynchronous payment status updates: charge succeeded, payment failed, dispute created. Webhook handlers must be idempotent because the same event can be delivered multiple times; they must process events in the correct order despite out-of-sequence delivery; and they must handle events for payments initiated in a previous application version. These are not edge cases; they are regular failure modes in production payment systems. A queue-based webhook processing architecture with deduplication and dead-letter handling is the standard approach.

Saved Payment Methods and Vaulting

Consumer and subscription applications require saved payment method management: storing tokenised card references from the PSP, presenting saved cards at checkout, handling card expiry via account updater services, and managing default payment method lifecycle. PSP-side vaulting keeps card data entirely within the PSP, which is the correct approach for PCI scope minimisation. Synchronisation between your user model and the PSP vault requires careful error handling for cases where the PSP request succeeds but the local update fails.

Our payment app development teams build and integrate across the full spectrum of payment infrastructure:

Payment Service Provider (PSP) Integrations

We integrate with Stripe, Adyen, Braintree, PayPal, and regional PSPs across APAC, EMEA, and the Americas. Integrations cover card payments, bank transfers, direct debit, and alternative payment methods. Our teams handle both server-side API integrations and client-side SDK implementations, with proper tokenisation flows to keep card data out of your systems.

PCI-DSS Compliance Architecture

PCI-DSS compliance is not an afterthought in our payment builds. We design card data flows from the ground up to minimise scope: network segmentation, tokenisation strategies, and audit logging are built into the architecture rather than retrofitted. Scrums.com teams have delivered PCI-DSS Level 1 and Level 2 compliant platforms for payment facilitators and marketplace operators.

Apple Pay, Google Pay, and Wallet Integrations

We implement Apple Pay and Google Pay across iOS, Android, and web, including domain verification, merchant identity certificates, and payment sheet configuration. For wallet integrations requiring dynamic shipping rates or coupon validation, our teams build the server-side session endpoints that drive the checkout flow.

Technology Stack for Payment Applications

PSP SDKs and Client-Side Libraries

Stripe Elements and the Stripe.js SDK for card capture with SAQ A/A-EP PCI compliance; Adyen Web Drop-In Component and native iOS/Android SDKs for multi-method acceptance; Braintree Drop-In for PayPal ecosystem integrations. Apple Pay is implemented via PassKit on iOS and the Payment Request API on web; Google Pay uses the Google Pay API. Both wallet integrations require merchant registration with the respective provider before they can be activated in a production environment.

Backend Payment Services

Node.js or Python FastAPI for payment service layers: fast webhook processing, low-overhead PSP API calls, and simple horizontal scaling. Java or Kotlin Spring Boot where the payment service is part of a larger JVM-based backend. All backend payment services must implement idempotency keys on PSP API calls to prevent duplicate charges on network retry, which is a requirement enforced by Stripe, Adyen, and most major PSPs.

State and Event Storage

PostgreSQL as the source of truth for payment state: order records, payment intents, refund records, and dispute records. An append-only event log table captures every state transition with timestamps for audit and customer support workflows. Redis for rate limiting on payment endpoints to prevent credential stuffing attacks on card entry flows and account creation paths.

Subscription and Billing Management

Stripe Billing or Chargebee for subscription management where billing complexity justifies a dedicated platform. Custom subscription scheduling for simpler recurring billing use cases. Dunning sequences implemented as scheduled jobs with exponential backoff retry intervals and configurable customer notification templates at each retry stage.

Fraud and Risk Signals

Stripe Radar or Adyen RevenueProtect for PSP-level fraud scoring on card transactions. Device fingerprinting via Sardine or ThreatMetrix for velocity checks and device reputation scoring on payment initiation flows. 3D Secure 2 for SCA compliance under PSD2; frictionless authentication for low-risk transactions reduces checkout abandonment compared with challenge flows and should be implemented with exemption logic for eligible transaction types.

Compliance and Security in Payment App Development

PCI-DSS Scope Management

PCI-DSS compliance scope is determined by how cardholder data flows through your system. The primary engineering objective in payment app development is to minimise scope to the lowest applicable SAQ level. PSP-hosted payment pages achieve SAQ A; PSP JavaScript components on your page achieve SAQ A-EP; custom card form elements that post data to your server require SAQ D, which mandates comprehensive security controls across the entire application infrastructure. Most product companies should target SAQ A or A-EP through PSP client-side libraries unless there is a specific product requirement that cannot be satisfied within those constraints.

Strong Customer Authentication and PSD2

SCA requires multi-factor authentication for online card payments within the EU and UK under PSD2. 3DS2 is the primary mechanism; the distinction between frictionless and challenge flow matters for checkout conversion. Certain transaction types qualify for SCA exemptions: low-value transactions below 30 euros, trusted beneficiary transactions, and recurring transactions after the initial authenticated payment. Exemptions should be claimed where eligible to reduce friction. Payment initiation services and account information services under PSD2 require FCA or national competent authority authorisation, or a partnership with an authorised PISP or AISP.

AML and Transaction Monitoring

Payment applications that hold funds or facilitate transfers between third parties require AML controls proportionate to the risk profile of the payment flows. Transaction monitoring rules should be designed with compliance input to ensure coverage of the patterns relevant to your specific payment use case. Automated monitoring must produce SAR-ready outputs and maintain complete audit trails. Customer due diligence and enhanced due diligence workflows for higher-risk customers must be integrated into the onboarding journey and triggered by risk scoring events during the account lifecycle.

Consumer Protection and Dispute Handling

PSD2 in the EU and UK and Regulation E in the US provide consumers with rights to dispute unauthorised and incorrectly executed payment transactions. The application must support a dispute intake flow, an internal investigation process, and timely response to the PSP or card scheme within mandated timeframes. Chargeback management requires integration with PSP dispute APIs to retrieve order and delivery evidence automatically for dispute submissions, reducing manual operational overhead at scale.

Why Work With Scrums.com for Payment App Development

Payment infrastructure is one of the highest-stakes areas of software development. A misconfigured webhook, a missed edge case in retry logic, or a compliance gap can translate directly into lost revenue, chargebacks, or regulatory action. Scrums.com provides dedicated payment engineering teams who have shipped production payment systems and understand what failure modes look like.

Our teams work across the full payment stack: PSP integrations, ledger design, reconciliation pipelines, fraud signal ingestion, and subscription billing engines. We bring that breadth to engagements whether you are building a payments product from scratch, adding payment capability to an existing platform, or migrating from a legacy processor.

For compliance-sensitive builds, we engage your compliance and legal stakeholders early. PCI-DSS scoping decisions, data residency requirements, and open banking regulatory obligations all inform architecture choices that are expensive to change later. Getting these right in the design phase is a core part of how we work.

Engagements start in 21 days. Start a project with us and we'll scope your payment build with you.