If your shopper doesn't follow your instructions, they'll quickly credit you.
If your shopper buys more bags than they give you, then you're stuck with the bill.
A shopper could buy ten bags for themselves, give one to you, but you're on the hook for all the rest.
I don't care that the bags aren't that expensive. If I got one bag, you charge me for one. It's not my problem if your shoppers use your card to pay for their own purchases.
Bunch of thieves.