I mean, that's one of the easier problems to solve. Basically make them a Roomba that has a set flight plan where it returns to its charging station at the end, and have enough that they can swap out and still keep coverage.
And depending on how it's made, powerless gliding is an option. They already use gliders in some places for speed limit enforcement.
You mean returning. They had it before they started forcing you to use Walmart Pay.
I know because I had a cart full of groceries and no wallet when they disabled it.