In the US, you are required to pay taxes on your income. If the IRS takes an interest in you and you are not able to prove in court that you properly paid taxes on the income you turned into your assets, they will take all your money.

None of this has anything to do with the format of any invoices. If you wrote a receipt on a piece of toilet paper, that's fine as long as you can prove that you then sent 30-50% of the money you received to the US government. I believe this is more generally a feature of common law legal systems which prioritize honest intentions over box-checking.

Whatever other requirements exist in other countries are not really a US business's concern, unless those countries start turning their merchandise away at the border. In any case, expecting a random payment processor to act as world paperwork policeman for the EU is hilariously ridiculous.