One possible dan lekaf zechus: In the novel Chocolate Liquor the main character works in a “supersave” type of grocery. At the end, her newly married daughter and sil come up to her checkout with a cart full of groceries way beyond their budget. “Did you win the lottery?” No, together with five other couples, they are making sheva brachos for a friend and volunteered to do the shopping with the pooled funds.
Granted, life is not a novel. It may be farfetched,but perhaps your neighbor is doing something similar and will be reimbursed or was given the money upfront and since it’s groceries, used her card? (Whether she has to pay in cash in that case, i don’t know.)