Picking an invoicing API is a decision you will live with for years. These are the questions that separate a demo-friendly product from one that survives audit season.
The checklist
- Which countries and formats are live today, and which are promised?
- What happens when a tax authority is down: queue, retry or fail?
- Are requests idempotent, and how are duplicates detected?
- Can you replay webhooks and fetch the full history of an invoice?
- Who is the legal sender, and can you onboard your own entities?
- How are certificates and keys stored and rotated?
- What does a sandbox cover compared with production?
Read the failure modes
Happy paths look identical across vendors. Ask to see what a rejected invoice looks like, how errors are coded, and how long a stuck document sits before anyone is alerted.
Finally, test with your ugliest real data: credit notes, mixed VAT rates and customers with missing identifiers. The way a provider handles those tells you most of what you need to know.