What really used to confuse (some of) them is to give them $20.03. Maybe not so much today where registers calculate the change, but in the old days it did.
The problem is that there is almost zero market for a lot of the "recyclable" stuff, certainly for most types of plastic other than 1 or 2. Even glass in some cities/towns is now considered trash.
Much of the problem comes from so-called single stream recycling. Easier for residents who can't be bothered to sort, but the cost of sorting takes away what little revenue comes from the sale. That and the fact that more and more third-world countries no longer accept our trash.