If you live in the Midwest, veterinarians recommend heartworm prevention monthly, and have also begun to recommend flea and tick prevention just as often. For one pet those medicaitons can be expensive; for more than one that's a sizable chunk of money spent on a necessary cost.
While purchasing medications from an online retailer may seem like a good idea, they often don't carry the same guarantees as identical products sold by veterinarians, and could potentially have defects or expire sooner than you could possible use them.
The best of both worlds option is to ask your vet to price match! Many offices need only a print out (or even a cell phone screen shot) to match the savings you would get with the online retailer, but with the added bonus of the manufacturers actually standing behind their products.
Next time you're at the vet, take a look around for signs advertising price matching. Even if you don't see anything obvious, what is the harm in asking? There are potentially hundreds of dollars in savings waiting for you!