They are not the cheapest--in fact I can get certain meds from my vet for much less (like heartworm.) And even when my vet is higher, he will match any price I find online and there are no shipping charges.Your best bet in to shop around online after you talk to your vet.
My vet's prices are as good as 800-pet-meds & I know if there is ever a problem with a med my vet will be responsive esp if the med needs to be returned. Also, there are numerous documented reports that some online human meds have been found to be inferior substitutes, not the same purity, dose etc so meds for animals may not really be same drug your vet would give you either. I know that with some online contact lenses, the lenses sold by some distributors are not manufactured in the USA like the ones available thru an Optometrist so quality control can be inferior. Even worse, some online sellers buy out expired contact lens inventories then repackage them in other countries - they then can claim they were made in the USA & give them a new expiration date based on the new packaging date! So... I prefer to know the person handling my meds & my dog's meds too.