You misunderstood me, I didn't imply that you need a VAT number as a seller company (as a US company you can't even have it), but just that you should be able to produce a simple invoice PDF with your address, buyer address, buyer VAT (if needed) and a sequential number to provide a paper we need to properly add payment into the accounting books.

And yes, I know that VAT isn't required piece of information on all invoices, but it WILL make the payment go smoother through the accounting departments which is what you want ;)