What a great question. I use Revolution and the price for me has gone up. I asked an Internet friend who is a vet about using discount pet med services and she said that she was not comfortable with them because she does not know the control processes in place for storage and temp etc. She also indicated concern about counterfeit drugs from some places. Of course as a Vet I am sure that she would want people to order their drugs from Vets...but what keeps your vet from ordering from one of the discount places?
Right now I am paying a higher price, but only because I am scared to death of heartworm medication failure. ( Dog getting heartworms.)
You bring up a very good point regarding the heartworm medication. The companies that make the major brands will pay the cost of heartworm treatment if your dog gets heartworm while on the preventative. This guarantee is only valid IF you purchase your preventative from the vet and the company knows the medication was handled properly. There are some other factors such as owner compliance and yearly blood tests (which help check for owner compliance) but this is not a medication you want to mess around with--as you have stated.
I know that the thread is basically about flea preventative, but this is good information to have. It is also one reason that some vets are hesitant to write prescriptions to online vendors for certain medications.