I'm a nonresident alien in US and I need to invoice a US company due to service. The company can provide W8 or W9 forms to proceed with the payments, but I need to provide an invoice confirming that I'll be paying all income taxes. Is this invoice "automatic" when I use this W8 form? If not, how can I provide this invoice?
You should ask the company you are working with what they require in an invoice. One thing is certain, invoicing will not simply be "automatic" because you file a W8 or W9.
At a minimum, your invoice should include your name (or company name), contact information, a break down of the goods / services provided & prices + taxes, a total amount, the date, payment terms (e.g.: due on receipt, 2/15 net30, net 30) and please... for the sake of their bookkeeping and yours... each invoice should have a unique identifier (not the date), even if it is something as simple as "Invoice#1".