Check the PetRx section. They price-match many competitors and offer a generic version (PetArmor) with the same active ingredient (Fipronil) for much less.
Often the gold standard for convenience. They offer a 35% discount on your first "Autoship" order and 5% thereafter.
Online specialized pet retailers usually beat local vet prices because they buy in massive bulk.
Usually has the lowest "per dose" price for a 3-pack or 8-pack. You do not always need to use their pharmacy; it’s often stocked in the pet aisle.