We actually use Stripe for our own invoicing right now. We may be based in Toronto, but we're with Silicon Valley Bank. I'm in SF right now and was just visiting the Stripe office this morning. Interesting things to come, I promise :)
How did you guys get a US tax number? Or are you using stripe as a personal account (SSN only)?
You can get a US EIN as an international business, just go to the site Stripe points you to and there are instructions for non-US businesses. As for the SSN, we were able to talk to the guys at stripe and get our account active without this.