Progresso is clean and friendly.
The pharmacies there are all going to be about the same price so there's no need to shop for bargains, in my opinion.
Take the cash.
Park on the US side in the parking lot and walk across.
Walk down the right side of the bridge and return on the opposite side of the bridge.
You probably don't need a prescription, but if you do, the pharmacy can recommend a doctor close by to write one for ten bucks. I've gotten a lot of prescription drugs and never had a prescription.
I buy a year's supply when I go. The men at the checkpoint look for hard drugs, not pharmaceuticals.