Payment processing question for Canadian business
I don't think you should have a problem using Google Checkout or Amazon Payments with your Canadian company. Check with those services which documents they require. Chances are they would want you to present them with an EIN, which you can obtain for your Canadian company without registering it in the USA.
In case you would decide you need a company in the US you might simply register it as a non??US person. Finally, if they require Social Security Number of one of the owners (for credit history verification or any other reason) then you would need a US partner, but I doubt they require it.