There's a cashier in a local Shoprite who has earned more then a few free trips because she scans her card (or a family members, not sure which) for the Continental miles. She scanned it once when I was shopping with too much stuff for the self service line, I asked her to scan mine instead for the miles and then she said that's why she scans hers whenever she doesn't think the customer has a card.

I smiled and said something like "Like the one I'm holding here right in front of you"

Two of the three chains in this area will find you a card to scan if you need one, though I do most of my shopping with the check out as you go carts or the self service asiles when offered so I don't fuss with the cashiers much anymore.

By the way, a great random tip I learned here on FT -

http://www.justoneclubcard.com/

You can put up to eight of the shoppers cards on a business sized card to stick in your wallet (or 16 if you do two of them, etc) so you always have the discount code even if you don't carry the keys into the store. Works great.