I'm retired. I still always pay cash for things so I can see track of my expenditure (unlike Pplspoet). I don't drink coffee but I will buy things at a small shop when I feel like a snack or cold drink. You have to run a viable business so I expect that any place I go (for anything really) provides what I want at a reasonable price with good service. If you unhappy customer can buy a coffee a lot cheaper elsewhere then you are in trouble. But it sounds like you are being reasonable to me.
Cheers
Cheers