Why is Propanolol More Expensive With PDP Than GoodRx?

I’m a rookie on how Medicare Part D prescription drug plans work, in which I put my mother on one.

My mother said she used to get her propanolol (that may have been when she was on an Advantage plan; she switched to Medigap) very cheaply she says. But the switch to Medigap required getting the Part D (she did have that before a switch to Advantage, but she was paying high premiums on that previous PDP). Now under the current PDP she pays a $20 monthly premium, has a $480 annual deductible, and $7050 TrOOP.

Propanolol should be almost as cheap as dirt. When she went to get a 90-day supply of 60 mg tablets, the network pharmacy said her charge was a whopping $296! I checked on GoodRx for what they would charge and it was $40.

What’s going on here? Is the insurer in league with drug maker to soak up the deductible cash (splitting it between them) and then I wonder what the co-pay/co-insurance will be once the deductible is reached? I urged her just to use the GoodRx coupon on that cheap drug and then maybe the PDP will come in handy in case she has to have more expensive drugs (for example, she may need drugs to treat lymphoma)

Anyway, does it work this way? Can she pay outside of use of insurance for cheaper drugs and not worry about applying them to her PDP deductibles and TrOOP totals?