Well, if you were looking after someone who ran a small shop, then it wouldn't be out of order for you to buy from the shop when you visit, would it. However, if you can, try to make any transactions outside of your working hours.
We council employees have to be very careful these days. If there aren't fellow employees out to get us, then there are clients and their families who are serial complainers. We have to take advice on every little thing we do. It wouldn't be a bad idea to call your HR department and ask for their take on the matter. Your boss doesn't need to know,