Why can’t I use my points AND a coupon at checkout?

“Half the time it depends on who’s working that day or which way the wind’s blowing. The rules seem made up as they go along.”

That’s honestly how it feels at the pharmacy too—sometimes you can stack a manufacturer coupon with points, sometimes you can’t, and there’s no clear reason. I wonder if stores are just worried about people gaming the system, or if it’s more about keeping things simple for their staff. Has anyone actually gotten a straight answer from a manager about why these policies are so inconsistent?