When customers visit the cart.php page, the products to be displayed are those belonging to the default product group. The default product group can be changed so that different products are displayed on the cart.php page:

Begin by navigating to Setup > Products/Services > Products/Services

The default product group is defined as the group which appears first in the product list:

The default product group can be changed by dragging the desired product group to the top using the arrow icon next to the Group Name:

A green success message will confirm the changes have been saved automatically: