As far as I'm concerned, and from what I've bought elsewhere, it never comes up.
Seldom do I get an invoice. Usually a PayPal receipt is the main thing. Some sites let me print an invoice if I want, but I never bother, if I get something from PayPal. Never an mention of a tax number, but there may be an EU law.
I've sold hundreds of thousands of dollars on line and never used any invoice beside the generic one sent by the shopping cart.
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