I don't need strangers to pay for my groceries either, but I think if they did (assuming it wasn't for a social media stunt) I'd accept it gracefully and try to pay it forward somehow. Maybe offer to pay for the next person, or use the money I'd have spent to buy those prepared bags for food pantries some shops offer.

It's feels good to genuinely do something nice for another person and I wouldn't want to take that from them. It's also nice just knowing that not everyone in your community is only looking out for themselves. Might as well take the opportunity to do something for someone else