Requirements for invoices in US for http://rechnung-plus.de/

Hi,

I'm European developer of the time-tracking and invoicing system Rechnung+ and now I want to implement English invoice templates for US users.

I want to know which rules are there in US regarding the look of the invoice, the numbering of invoices, required company details like VAT-ID (what's the US name for VAT-ID and is it required on the invoice?)...

Here's the screenshot of the direct translation of German invoice into English. Please let me know what shall be changed/added.



Thank you for your help in advance,
Slawa.
http://rechnung-plus.de/