Ask for the details you need
Ask for the legal company name, billing address and tax number as they should appear. Confirm them back before you issue anything.
Send the right document
Issue the invoice from your billing system rather than writing one by hand, and make sure it carries your own required details. If you are unsure what is required for a country, ask your accountant.
Correct mistakes promptly
If a name or number is wrong, reissue the invoice and say which one replaces the old one. Do not just send a second copy without explanation.
Do not give tax advice
Explain what you charged and what appears on the invoice. For questions about what the customer can reclaim, suggest they ask their accountant.
Make it self-serve
Let customers add their tax details and download invoices from their account. Most requests disappear once they can do it themselves.
Track what is asked
Tag tax invoice tickets and review them monthly. A rise after a pricing or checkout change usually means the details are missing at purchase.