> All they would have to do to support this is add a checkout field "VAT number" that shows up on a pdf invoice.

If only it would be that simple :)

In EU you have different procedures for B2C and B2B transactions. For B2B you need to verify the VAT number in VIES system and it’s not responsive like 50% of the time. I swear Germans literally turn off their servers when they go to sleep. If a customer provides a VAT number the flow might take even 12h+ to verify it. If you can do that verification you can use 0% VAT rate but if not you need to use a different VAT rate.

For B2C you need to support several scenarios: if company is outside of EU it needs to register for IOSS, if it’s a EU company that sells to other EU countries it needs to register for OSS or in each EU country for VAT separately but also a mix of both is possible. You can decide to no register to OSS special procedure but then there’s a sales limit before you have to register and you need to track it. Otherwise, you need to maintain special OSS registry with sales records and three pieces of proof that customer is based in the member country. Some EU countries have XML invoices (Italy, Romania, Germany soon) or mandatory invoice APIs (Poland), of course there’s actually no common EU standard so it depends on where the company is based.

Finally you need to choose a VAT rate for that country and they also change occasionally, e.g. Slovakia, Romania and Estonia all changed their highest rate just last year.

This is the bare minimum you need to support. There’s a lot of edge cases, e.g. it matters what country you actually ship from, and if you use e.g. fulfillment there are special procedures for that as well, or if you resell in B2B there are chain transactions which have their own set of spaghetti rules.

Much of that is at least for my company handled by our accounting company. We just print the correct VAT on the invoice, and report the same VAT to the accountant and they take care of the rest. The shop/payment processor etc doesn't need to be integrated to any of it. Though I have to post-process Stripe's reports, as they refuse to include the used VAT rate in there, despite them knowing it. Stripe does try to sell the tax service to us, but I refuse.