Frontline’s active ingredient (fipronil) is suspected by some vets of no longer being effective in some areas due to resistance. It’s one of the older flea prevention products, so fleas have had many years to develop resistance. The manufacture disputes that resistance is a problem. However, the shelter vets in my city stopped using it in the shelter because they perceived it as being ineffective. We also had to stop using fipronil products for our foster dogs because they did not prevent fleas. Advantage worked great though, as did newer products like NexGuard and Vectra3D. If Frontline still works in your area, there are many generics for it now that Fipronil is off patent. YMMV.
Also, I am very skeptical of “weirdly cheap” dog meds on Amazon due to Amazon’s severe problem with counterfeit and gray market goods. The risk of fake products is pretty serious. I buy from Vet-VIPPS certified pet pharmacies, which means I am guaranteed to get FDA-approved meds just like the vet sells. I especially love the customer service on ValleyVet.online. KVSupply.online is also pretty good. Costco and Walmart pharmacies also sell many flea and heartworm products for dogs.