Walmart has had the cheapest prices for prescription drugs for years. A 30-day supply is only $4 and a 90 day supply is $10 if you go into the store. So the only difference in OP is that they’re willing to ship a 90 day supply. I couldn’t figure out if shipping is free.
If you have prescription coverage they’re supposed to charge you whichever is the cheaper price, your cost with insurance or the WM price.
Pharmacies sometimes have a minimum fill charge. So if you need e.g. 10 Penicillin and the cost of the drug is only $2 but their minimum fill charge is $4 you pay $4. Minimum fill charge covers the cost of processing, the bottle, label printing, etc. The meds on WM’s low cost prescription list are prepackaged in blister packs on cards. They just pull one box for 30, 3 boxes for 90. No need to count pills. It saves a lot of time and decreases the chance of error.