Some clients and companies allow using coupons, some do not. I do a lot of grocery stores. One shop I have 2 required purchases out of a short list of items and am allowed to purchase NOTHING ELSE during the visit. Another type of grocery shop will reimburse me up to a limit and has no specific requirements of what I purchase except that I must have a bulky item to put in the bottom of the cart to make certain that the cashier and/or checker are watching the bottoms of the carts for merchandise that customers 'forget' to pay for. Another type of grocery has a limit and usually 2 specified purchases but the rest of the reimbursement I can use however I want.
You need to read the instructions for the shop and see what is forbidden. My main grocery shops do not prohibit coupons, so I slowly introduced coupons, and when there were no complaints stepped it up. Between the BOGOs and the coupons and the reimbursements I have paid very little out of my own pocket for groceries over the past few years.
At the same time, I used coupons at one shop because they were not expressly forbidden, and quickly got back a note saying that coupons were not allowed and acceptance of the shop by the client was in jeopardy. The job was accepted, but no more coupons there!