Building a Unified Payment SDK

About the Project

To simplify and secure payment processing across a large-scale transportation platform, we developed a proprietary Payment SDK for both iOS and Android. The SDK enables seamless payments through a variety of external devices and services, including POS terminals, QR codes and NFC/contactless payment technologies, all managed through a single, unified interface.

Designed for flexibility and scale, the SDK integrates with both the client’s own and various third-party PCI DSS – compliant payment provider servers, supporting secure, one-step transactions and powering millions of payments monthly.

Business Challenge

The platform needed a unified solution capable of interacting with multiple types of payment devices and services, each with different connection types, hardware requirements, and interaction flows. Key requirements included:

• Unified support for POS terminals, QR-code-based payments and NFC/contactless technologies like Apple Tap to Pay, each with its own underlying communication protocol and/or API
• Seamless handling of Bluetooth and Wi-Fi pairing flows
• Secure communication with both client and third-party payment processing servers
• Real-time device discovery and readiness status detection
• Compatibility with device-specific UI flows, including fully custom interfaces when required
• Handling of transaction edge cases, including declines and time-limited refunds

Contributions

We designed and implemented a modular, plugin-based Payment SDK to abstract the complexity of working with various payment devices and services behind a clean, consistent interface:

• Defined a standard protocol for all supported integrations, covering connection, configuration, payment processing, tipping, refunds, and disconnection
• Enabled runtime discovery and availability checks for payment devices and services
• Developed and maintained individual plugins for each payment integration, OS terminals, QR-code-based payments and NFC/contactless technologies like Apple Tap to Pay
• Handled Bluetooth and Wi-Fi pairing flows through built-in connection workflows
• Supported both interactive and non-interactive payment scenarios, with conditional UI when required
• Implemented transaction reversal logic, allowing time-limited refunds of the last transaction

Reusable & Scalable Solution

The SDK was built as a scalable and reusable solution, not something made just for one project. It can be easily adapted for future projects that require secure, device-integrated payments.

• Modular plugin architecture makes it easy to add new payment devices or providers
• Unified protocol ensures consistent behavior for connection, scanning, payments, and refunds
• Developer-friendly design with clear abstractions allow quick onboarding and minimal integration effort
• Flexible and suitable for a wide range of use cases beyond the original scope

Key Expertise

This project reflects our deep expertise in payment technologies and our ability to deliver complex, production-ready SDKs built for scale and flexibility:

• Experience with a broad range of integrations, from low-level POS terminals using binary protocols to high-level contactless solutions like Apple Tap to Pay
• A unified SDK architecture that supports both proprietary and third-party payment providers through a single interface
• Proven ability to deliver secure, PCI-compliant solutions that process millions of real-world transactions every month
• Focus on developer experience by offering clear abstractions that remove the need for app teams to manage provider-specific complexity