A recent Wal-Mart order sort of exposed how they do it. The order is picked and packed at the store. Then the driver picks yet pIcks it up and delivers it.
In one delivery I got something I did not order, Prestone GM coolant. On a subsequent order we got other things we did not order. E.g. White Castle sliders, hot pockets pizza, and sausage egg and cheese croissants. The driver was funny she told me keep them and try them. I showed her the coolant asked if she wanted it.. So in my book it was an equitable trade at the