This sounds a lot like Real Time Inventory (RTI) and RDIF technology. The RDIF readers know what’s physically in the store. When you walk out with merchandise, it’s no longer in the store and no longer a part of inventory. How they scan a cart walking out the door is easily worked out, and the billing system is already in place. But in a nutshell, this is RTI and will lead to very lucrative shrinkage savings.
No need. By the time you leave, they already know what you have, and your departure simply prompts them to mark your stuff sold and total up the bill.
You might find this document informative. The abstract:
This disclosure describes a system for automatically transitioning items from a materials handling facility without delaying a user as they exit the materials handling facility. For example, while a user is located in a materials handling facility, the user may pick one or more items. The items are identified and automatically associated with the user at or near the time of the item pick. When the users enters and/or passes through a transition area, the picked items are automatically transitioned to the user without affirmative input from or delay to the user.