If you want to sell services to large companies--even low cost ones--you need to offer invoice payment. At a big company, the corporate credit card is for meals and travel, not for paying vendors.
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.
Do you have an example of a SaaS operation doing this well?
Hmm. Sounds like a complete and utter horribly painful schlep. A schlep that puts you right in the path of the money. Interesting.
I think you meant sole, but I had to laugh at this meaning as well in the whole scheme of dealing with finance departments.
(I think I picked this idea up from Patrick McKenzie -- another one is offering SLAs to expensive clients).