I don't know what's been tried in a real retail market, but I remember Walmart or perhaps just designers trying all sorts of ideas from carts that scanned them to people using their phones. It starts to get really complicated when people want to remove things or you have multiple people putting stuff in carts (eg. children sneaking items in)

I'm sure it's solvable, it's just a question of whether it's worth the cost for shoppers and the stores.

There's another challenge that the self-checkout poses where people cover the UPCs with UPCs for cheaper items (either self printed, or by taking off stickers from cheaper items). Most of these RFID things are also stickers so that also becomes a problem.