The second scan at exit is done one after the other quickly with the employee's hand scanner without handling again. Then just wheel the lot out to your car, unloading it there. Eliminates a lot of handling or waiting, and also the time lag of fiddling with your money card.
Accurate for both seller and buyer, and hard to beat for convenience. You won't get the feel of it by staying home yourself.
The setup at Walmarts doesn't at all work as well.
I have done Wal*Mart, and agree that it is less than ideal.