Hi Jim,
ecommerce package is an early OpenACS package designed to provide a commonly needed set of shopping-basket features.
payment gateway packages are set-up to be used with any ecommerce-like package.
payment-gateway package integrates an ecommerce-like package with a payment gateway package by creating an ACS service contract via the acs-service-contract package; Most of payment-gateway's documentation is in the payment-gateway-sc-create.sql file and the docs at https://openacs.org/doc/acs-service-contract
From what I understand, the system is designed to work with variants of ecommerce and gateway packages with different features to handle different commerce conditions.
