I wonder how an invoice to an US customer should be. I live in Europe: in my country no business can pay a good or a service without a legal proof, which can be a receipt for a low sum but generally is a proper invoice which must have well fixed characteristics. If you don't get such an invoice you get in trouble, so as the people who sold you the good or the service without issuing it.
Which are the rules to follow? What about VAT?
Now I'm puzzled. I would like to work for the US market but I don't know how to do things under US law (and I confess I'm also struggling to understand what I should do to comply with the income tax rules in my country......).