FastAPI Shipping Label Queue Integration

This project centers on extending an existing FastAPI back-end so it can pre-generate shipping labels, place them in a queue, and push them into the Fulfil.io platform that runs our fulfilment workflow.

Core flow
• A request comes in through FastAPI.
• The service immediately generates a PNG label (no other formats needed) and stores it in a queue for downstream processing.
• Once queued, the label data is sent to Fulfil.io so our team can bulk-print everything from within that system.

Key points to keep in mind
• Labels must be produced in batches large enough for smooth bulk printing.
• Queuing should be resilient—Redis, RabbitMQ, or a similar broker is fine as long as retries and failure handling are covered.
• The codebase must remain idiomatic FastAPI (Pydantic models, dependency injection, async awareness).
• Fulfil.io’s API is the sole integration target; no FedEx, UPS, or DHL endpoints are necessary.
• Output files are PNG only, delivered via an S3 bucket or another pluggable store that Fulfil.io can pull from.

Deliverables
1. FastAPI endpoints (generate label, fetch status).
2. Queue worker script with clear run instructions.
3. Fulfil.io integration module (auth, label push).
4. README covering environment variables, setup, and a sample bulk-print run.

Acceptance criteria
• Generating 100+ labels in one call completes without timeout and every label reaches Fulfil.io.
• Failed pushes automatically retry up to three times and surface clear logs.
• A simple pytest suite confirms the above behavior.