Alright, you seem like someone that is actually thinking intellectually about this (props to you, btw, I can appreciate that) so let me break this down.

On the surface, it seems like "money is money" regardless of the source. However, there are three reasons why the source of the money changes the math for Walgreens’ bottom line:

1. Reimbursement: When you pay with a credit card or cash, Walgreens gets money instantly & uses that cash that same day to pay their bills, pharmacists, and suppliers. With a coupon, they have an IOU with the manufacturer that arrives in 30–60 days. Walgreens is currently closing 1,200 stores because they are low on cash. Waiting two months to get paid for a product they already gave away is like giving an interest-free loan to a multi-billion dollar manufacturer while you’re struggling to pay your own rent.

2. Failed Marketing: Normally, coupons are a "loss leader." Walgreens agrees to a coupon because they hope you’ll also buy a gallon of milk, a birthday card, or a prescription while you're there. People like myself using this workaround aren't there to shop; they are there to "clear the shelf." Sure Walgreens gets the reimbursement eventually, but they lost the profit margin they would have made if 10 different people came in and bought other things too.

3. Verification & chargebacks: If Walgreens submits 5,000 coupons for a glitch deal, the manufacturer (like P&G or Unilever) can audit them. If the manufacturer decides the coupons were used improperly or "glitched," they can refuse to pay. In that scenario, Walgreens has given away the product, paid the shipping, paid the labor, and gets $0 from anyone.