Add Third-Party Integrations

I already have a working application in production, but it now needs fresh capabilities. My main goal is to extend its functionality by integrating selected third-party services (APIs, payment gateways, or SaaS tools that make sense for the platform).

What I need from you:
• Review the current codebase and architecture so the integrations slot in cleanly without breaking existing logic.
• Design and implement the new API connections, including authentication flows, error handling, and rate-limit safeguards.
• Expose the new functionality through clear service classes or endpoints that match our current coding style.
• Provide concise setup documentation and a post-integration checklist so future updates stay manageable.

The focus is on new features, but if you spot minor refactoring opportunities that improve stability or performance while you work, flag them and we can discuss. A solid grasp of REST/GraphQL, OAuth2, webhooks, and version control (Git) is expected.

Please share a brief outline of similar integrations you have delivered, the tech stack you prefer for this job, and your estimated timeline for a first working build.