It’s an interesting approach, as you wouldn’t enter a restaurant and ask what dish would make them the most margin. Or go to the post office to ask what packaging is cheaper for them to ship. In that respect I think asking what payment means costs more to the shop would break a social norm, so I’m surprised you file it under politness.
Otherwise shop owners have many many ways to signal how much they want you to do something or not. Wether the card reader is hidden under the counter or the most prominent thing when you need to pay could be a very clear signal for instance.
