What is the ruling on creating customer accounts in a stationery and photography supplies shop, where customers deposit a sum of money from which their purchases are deducted in exchange for a certain discount percentage on each invoice?

The funds that are at your disposal and that you guarantee to the client are considered a loan from him to you. It is not permissible to stipulate a benefit in the loan for the lender, because that is forbidden usury (riba). Scholars have unanimously agreed on the prohibition of every loan in which an increase or a gift to the lender is stipulated, because it transforms the loan from being a contract of assistance and a charitable act into a usurious transaction.