Custom Payment Gateway API Integration Services
Integrate Stripe, PayPal, Braintree, and custom payment APIs for secure checkout, subscription billing, automated refunds, and real-time webhooks. Built with automated testing, telemetry monitoring, and zero-data-loss failure recovery.
We move beyond basic tutorials to engineer production-ready payment architectures. Pair your payment engines with general API integration services, custom e-commerce platforms, mobile app builds, and custom web applications.
Production-grade payment infrastructure engineered for conversion and compliance:
- Starts From $300: Predictable fixed pricing, transparent delivery sprints.
- 100% Code Ownership: Private Git repository with TypeScript/Python source code.
- SAQ-A PCI Compliance: Zero raw card numbers on your servers. Minimal audit scope.
- Zero Data-Loss Queues: Redis BullMQ webhook retry queues prevent lost revenue.
Engineered for High-Stakes Payment Environments
Whether scaling recurring SaaS revenues, multi-vendor marketplaces, or high-volume storefronts, we build bulletproof payment pipelines.
SaaS Platforms & Subscription Apps
Automate recurring billing & dunning
Engineer tiered seat licenses, usage-based metered billing, upgrade/downgrade prorations, and automated dunning retry queues to stop churn.
High-Volume E-Commerce Stores
Express mobile checkouts & conversion
Deploy hosted Stripe Elements, Apple Pay, Google Pay, and localized payment methods (Klarna, iDEAL, SEPA) with instant webhook fulfillment.
Multi-Vendor Marketplaces & Platforms
Automated seller onboarding & payouts
Architect Stripe Connect or PayPal Hyperwallet workflows for merchant KYC verification, commission retention, and automated payout splits.
High-Ticket B2B & Invoicing
Secure bank transfers & ACH payments
Implement automated ACH direct debits, wire reconciliation, recurring enterprise contracts, and customized PDF invoice receipts.
6 Essential Payment Flows We Architect
Every business model requires specialized transaction logic. We design payment architectures scoped to your exact commercial rules.
One-Time Payments & Express Checkout
Hosted card fields (Stripe Elements / PayPal SDK), 3D Secure 2.0 (3DS2) authentication, Apple Pay / Google Pay express buttons, and instant receipt generation.
Subscriptions & Recurring Billing
Flexible billing cycles, metered usage tracking, tiered seat plans, trial period setups, upgrade/downgrade logic, and automated prorated calculations.
Refunds & Dispute Webhook Hooks
Automated partial and full refund processing, chargeback dispute event handling, and instant notification triggers to customer support systems.
Failed-Payment Recovery (Dunning)
Automated smart retry queues, expiring credit card notifications, customer self-serve billing portals, and graceful service access revocation.
Marketplace Payouts & Split Routing
Stripe Connect / PayPal Hyperwallet setup for multi-vendor payout splits, automated commission holds, tax withholding, and scheduled payouts.
Multi-Currency & Regional Localization
Dynamic currency conversion across 135+ currencies, localized payment methods (iDEAL, SEPA, Bancontact, Klarna), and automated sales tax/VAT compliance.
Production Security & Fail-Safe Architecture
Payment APIs handle sensitive financial transactions. Our engineering standards ensure your payment pipeline is safe, audited, and resilient to network failures.
Hosted Payment Fields (SAQ-A Scope)
Card credentials are captured directly inside Stripe Elements or PayPal iframes. Raw card numbers never touch your application server, reducing PCI DSS compliance to SAQ-A.
HMAC SHA-256 Webhook Validation
Every incoming payment webhook payload is cryptographically verified against your signing secret before triggering business actions or updating databases.
Idempotency Control Headers
Unique Idempotency-Key headers are attached to every transaction POST request, guaranteeing that network retries never execute duplicate charges.
Redis / BullMQ Retry Queues
Asynchronous background queues with exponential backoff buffer transaction events during target server downtime or HTTP 429 rate limits.
Audit Logs & Telemetry
Structured, PII-sanitized JSON logs with correlation IDs record every payment lifecycle state transition for 100% auditability.
Encrypted Secrets Management
API keys and webhook secrets are stored in AWS Secrets Manager or HashiCorp Vault with strict environment segregation and key rotation.
Want to learn how we engineer Stripe webhooks?
Read our technical engineering guide covering HMAC signature validation, raw body buffer parsing, and idempotency keys.
Supported Payment Processors & SDKs
We bring hands-on experience integrating industry-leading payment gateways across global and regional markets:
Stripe
Connect, Billing, Elements, Radar, Webhooks
PayPal & Braintree
Vault, Smart Buttons, Subscriptions, Payouts
Square & Authorize.Net
Omnichannel checkout, merchant account sync
Razorpay & PayFast
Regional Asian & African payment gateway APIs
Klarna & Afterpay
BNPL Buy Now Pay Later installment API flows
Apple Pay & Google Pay
Biometric 1-click mobile express checkout SDKs
Multi-Currency SaaS & Marketplace Payment Infrastructure
Processing $2.5M+ in annual subscriptions & vendor payouts across 14 currencies.
The Challenge
A growing SaaS platform suffered from duplicate card charges during transient network timeouts, dropped webhook events during subscription renewals, and 20+ hours/week of manual vendor payout calculations.
The Custom Solution
Codexify engineered a decoupled payment engine using Stripe Connect, Redis BullMQ retry queues, cryptographic HMAC SHA-256 webhook verification, and automated dunning recovery workflows.
Quantitative Results
- 99.99% Payment Sync Reliability
- 0 Duplicate Charges (Idempotency Enforced)
- 100% Automated Vendor Payout Splits
Our Payment Integration Process
Every payment integration follows a strict 6-step engineering workflow to ensure zero data loss and PCI DSS compliance.
Payment Scope & Compliance Audit
We audit your payment business model (one-time, subscriptions, payouts), regional currency requirements, PCI DSS compliance scope, and target gateway APIs.
Gateway & Security Architecture
We design hosted payment field layouts, cryptographic HMAC webhook listeners, Redis BullMQ retry queues, and idempotency key enforcement rules.
Sandbox Engineering & Webhooks
Our engineers build the payment middleware, integrating gateway SDKs, raw body webhook parsers, subscription engines, and automated dunning workflows.
Failure Scenarios, Dunning & Stress QA
We simulate network timeouts, 3DS2 card challenges, invalid webhook signatures, expired cards, and rate limits to verify zero transaction drop.
PCI Verification & Live Production SLAs
We verify clean SAQ-A PCI compliance, switch to live production keys, configure real-time Sentry and Slack telemetry alerts, and hand over 100% source code.
Tangible Payment Deliverables
Complete source code ownership, interactive OpenAPI specs, automated unit test suites, and production deployment scripts.
1. 100% Client-Owned Source Code
Clean, modular TypeScript, Node.js, or Python payment integration codebase pushed to your private repository.
2. OpenAPI 3.0 & Postman Specs
Complete documentation containing request/response schemas, payment webhook event signatures, and testing headers.
3. Automated Payment Test Suite
Unit and integration test suites using Jest and mock payment gateway endpoints to prevent payment regressions.
4. Deployment & Secrets Setup
Dockerized container files, AWS Secrets Manager templates, and CI/CD deployment pipelines for seamless releases.
Transparent Pricing & Delivery Timelines
Clear, fixed-scope payment gateway integration packages backed by our production-readiness guarantee and zero-hidden-fee promise.
Starter Checkout
Single gateway setup, hosted card fields & express buttons
- Stripe Elements or PayPal Smart Buttons setup
- Apple Pay & Google Pay 1-click mobile checkout
- 3D Secure 2.0 (3DS2) authentication flows
- Cryptographic HMAC webhook signature validation
- SAQ-A PCI compliance scope reduction
- 1 week post-launch warranty support
Standard Subscriptions
Recurring billing, tiered SaaS plans & automated dunning
- Tiered subscription plans & metered usage billing
- Automated dunning recovery for failed card payments
- Customer self-serve billing & invoice management portal
- Prorated upgrade, downgrade & cancellation logic
- Automated tax / VAT calculations (Stripe Tax)
- 2 weeks priority post-launch support
Advanced Marketplace
Multi-vendor platform payouts & split transaction routing
- Stripe Connect or PayPal Hyperwallet split payouts
- Automated seller onboarding & KYC identity flows
- Platform commission fee holds & payout schedules
- Multi-currency conversion across 135+ currencies
- Redis / BullMQ retry queue for webhook durability
- 1 month priority support & SLA monitoring
Factors That Influence Custom Payment Scope
Payment Gateway Count
Single gateway setup (e.g. Stripe Checkout) vs multi-processor routing (Stripe + PayPal + Braintree fallbacks).
Subscription & Dunning Logic
Simple one-time transactions vs complex tiered subscriptions, metered usage, and automated dunning recovery queues.
Marketplace & Payout Splits
Direct merchant checkouts vs complex multi-vendor payout split routing (Stripe Connect / PayPal Hyperwallet).
Multi-Currency & Regional Taxes
Single currency billing vs dynamic 130+ multi-currency pricing, localized payment methods, and automated VAT/GST rules.
PCI DSS & Auth Scope
Standard SAQ-A iframe integrations vs advanced custom tokenization & dedicated HSM security proxy architectures.
Ongoing Maintenance SLAs
One-time code delivery vs ongoing SLA monitoring for gateway API version deprecations and webhook secret rotations.
Ready for an Exact Fixed Proposal?
Schedule a payment architecture consultation and our lead engineers will provide a clear, scope-based proposal within 48 hours.
Related Payment Engineering Guides
Explore our engineering blueprints on Stripe integration, webhook security, PCI DSS compliance, and API pricing models:
Payment Gateway API FAQ
Common questions about our payment integration services, PCI compliance, and security.
Ready to Build Secure, Fail-Safe Payment Infrastructure?
Partner with Codexify Solutions to build PCI-compliant checkout sessions, recurring subscription engines, marketplace payouts, and Stripe/PayPal webhook listeners.