Authorize.net, basically just an API front for the real money handling, is only available through 3rd-party resellers. My best guess is that 37s's bank is one of those and is also providing the merchant account.

I know that you can just grab a merchant account and then find a reseller for Authorize separately, but it just seems easier to take the package deal from someone, and banks are safer than some random Authorize reseller on the web, IMO.