I'd call them and tell them the same thing you said in your thread, about the cost difference and ask if they made a mistake.
If they say no, then going forward, you can tell them you want to check the cost of the meds at the drugstore before buying.
My vet doesn't seem to charge more, at least not enough that I've noticed. Both she and the internist are willing to write scipts, for no other reason than the pharmacy is closer
.