Thanks, Univate... I called and spoke to one of the PaySimple folk, and learned the following: PaySimple has a 2.0 version and a 1.0 version; the 1.0 version utilizes the USA ePay payment module for Ubercart/Drupal. According to the Drupal module page, it's Ubercart 2.0 compatible.

Clients on PaySimple 2.0 who want to integrate can downgrade to PaySimple 1.0 -- it's "essentially a switch" on the backend; the only feature they lose is email invoicing, so if they don't do that then they can downgrade their PaySimple version w/o any other loss and come up on Ubercart 2.0.