An obvious question is: what is a pharmacy's incentive for this kind of discount?

This has puzzled me, too. Our Humana medicare plan gets us most of our mail order drugs for $0 or trivial amount.

For an immediate prescription we use goodrx to see what they have for a local pharmacy. Walgreens or Sam's Club or Kroger or Walmart. Some of the time the Goodrx price is cheaper than the Humana plan "local pharmacy" price. One in particular was $95 on the plan or $12 on Goodrx.

The lady at Walmart happily runs a compare to see which way gives us the cheapest price. I have NO IDEA how they would get the difference. They already have the drug in stock, they already paid for it. Do they get a reimbursement from Humana or GoodRX? Where to _they_ get the money?

We had one where Kroger w/Goodrx was the cheapest for a 3 month supply. The next time we went they told us that the previous Goodrx price was no longer and the new price was a LOT higher. So we went to Walmart.

We just recently got another RX card in the mail, unsolicited. "OptumPerks"

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Man oh man.
I just compared B12 (Cyanocobalamin), Optum vs. Goodrx.
Walmart $21.12 vs. $43.98

But.......we have noticed that sometimes Goodrx has a discounted low price on the first time you fill a RX and higher when you refill.