It would probably be fairly cheap for you, and profitable, to keep on hand several kinds of teabags and instant coffee. (Note: I do not drink coffee so I don't know if instant is awful or not.) All they need is a cup and hot water.
If you want to take a chance on foods that need to be sold within a few days you could try pre-made wrapped subs or sandwiches. Bagels usually go down well too. And if you're keeping any kind of freezer, ice cream bars/sandwiches, etc. (Although ice cream can make a mess in your store!)