No need for them to talk to your phone or a smart bag. They just funnel you out through a lane and scan them there if they're using RFID. They can also do this theoretically without using RFID at all if they can solidly determine what product you're taking off the shelf, maybe by using good cameras to see where you're reaching and if you're placing or replacing the item that's assigned to that slot. They've been tracking you since you entered the store so they know what Amazon account each person is associated with so they just combine the 2.

As for loose weighed items it doesn't look like they're selling those. /Everything/ in the promo shot of the store is already packaged and they look like they're going for more of a convenience store food source than a traditional grocery store.