I always ask for reusable bags if I can see that the custy has them in their cart. However, there is signage posted at the end of my cash - bright yellow sign with black letters, can't miss it - saying to place the bags first on the belt, unpack your Greenboxes (these bins that we also sell) and leave all enormously heavy things in the cart.
Yet... If they ignore me, then it's plastic for them and I refuse to repack the ones done before they hand over their bags.
I am a damn good packer. Even those plastic bags will stand up and are packed efficiently.
I have had custys bitch at me about how their bags get packed while they come over to my self scan and proceed to put a can on top of their chips, thinking nothing will happen.
Sigh. Even the stupidest of my coworkers wouldn't do that, and I've supervised most of them during their training shifts.
--Self Scan Queen