Our school shop is really a teaching resource rather than a 'shop'. The shop has simple stationary and some fruit juices (not fizzy sugared drinks) costing from about 50 cents to about $1. Children's parents may give a small stipend to the shop (typically $25 per term) which the children can use on a credit basis.
However, the real innovation in the shop is that the children are the shop keepers. They calculate the remainder of credit for each child using their own maths abilities. We pair an older and younger child together. We also have a teacher with the children to ensure that they get their sums right.
We feel that this is a way to make mathematics and maths skills more relevant but trying to get the children to use them in real world applications.