In my experience, if you send a proforma invoice, a W9, a federal tax id, and a soul sourcing statement, at the end of the signup form, many times a check will just show up in the mail. It is possible to do invoice payment without having a high touch sales process. You just have to preemptively provide all the stuff the AP department needs.
I really think 37 signals is leaving quite a bit of money on the table by not offering PO payments. I understand they don't want to do a whole bunch of customer hand holding, but it is possible to accept POs without ever talking to a customer.
Just dreaming here, but is it possible that this could be outsourced, the way you can outsource credit-card payments with Stripe? Have another organization invoice them, collect the check, and then deposit the money into your account, without you ever having to make more than an API call?
You don't have to customize anything. If you don't want them to call, you can add a sentence that you accept (or don't accept) foreign currency checks at a market exchange rate. That is about the only question foreign businesses ask.
I really think 37 signals is leaving quite a bit of money on the table by not offering PO payments. I understand they don't want to do a whole bunch of customer hand holding, but it is possible to accept POs without ever talking to a customer.