Drupal

Drupal Commerce is open-source eCommerce software that augments the content management system Drupal. Within the context of a Drupal-based site, Drupal Commerce presents products for purchase; walks customers through the checkout process; keeps track of invoices, receipts, orders, and payments; facilitates shipping and payment; and performs other functions needed by online merchants.

Excerpt from Wikipedia: Drupal Commerce. Content available under CC-BY-SA 4.0.