Fonoa vs TaxCloud

Fonoa

The API reference is served as Markdown with an OpenAPI definition on each endpoint page, and e-invoicing submissions are idempotent by transaction_id. API keys are issued by Fonoa staff after contact, no price, rate limit or changelog is published, and the only public terms date from May 2020 and cover older seller sites.

TaxCloud

A 30-operation REST API with a public OpenAPI 3.1 contract, llms.txt, Markdown docs and a 30-day trial that needs no card. The vendor publishes no rate limit numbers, no SLA, no API changelog and no official v3 client library, and the security page names no audit or certification.

Fonoa

1. Send the key in the Ocp-Apim-Subscription-Key header. Use the sandbox key at https://sandbox.fonoa.com and the production key at https://api.fonoa.com
2. Treat validations and e-invoicing submissions as asynchronous. POST, then poll the GET endpoint or wait for the webhook, and avoid tight polling
3. Reuse the same transaction_id on a retry of /v1/transactions. A transaction already processed is not processed again
4. Commit a calculation with POST /tax/v2/calculations/<uuid>/commit only when the sale is final. The commit adds it to revenue tracking
5. Ask the Fonoa contact to configure webhooks. There is no API for it, and always fetch resource_url as given in the webhook

TaxCloud

1. Send X-API-KEY on every call and put the connection ID in the path, as in /tax/connections/{connectionId}/carts. A wrong ID returns 404
2. Mark nexus states active in the account first. With none active the API returns zero tax and no error
3. Use a separate test connection. There is no sandbox, and orders on a live connection go into filing
4. Pass a stable idempotencyKey on refunds, and never convert the same cart twice. A second conversion creates a duplicate liability
5. Void a wrong order before 12:00 AM Eastern on the 10th of the month after its completedDate. Later requests return 422