Never buy drugs online. I'm a vet and the reason we don't endorse these pharmacies is that the source of the drugs is often unknown. The big manufacturers, Pfizer, Merck, Novartis, etc. do NOT sell to these companies, so the drugs are often purchased overseas and then shipped back to the US for distribution through online pharmacies. Who knows what happens to them in the intererim... and if you saw Dateline last night, they may not contain ANY of the actual medication advertised. Also meaning that if, God forbid your pet should have a reaction from a medicine purchased online, you're totally out of luck as the companies' guarantee is void. There has been lawsuit after lawsuit involving these companies. In order to sell medication, by law one must have a 'Doctor-Patient-Client' relationship. PetMed et al have never examined the animals and have been caught writing prescriptions for animals they've never seen. This is illegal in most states.
I've delt with these companies for years and let me tell you, they're not interested in your animal, only the money. The clinic where I work now refuses to deal with them, but the place where I worked before would occasionally write scripts for them (although why I'll never know). From time to time, we would get a request for heartworm pills for a dog who had been off them for some time. We would call the owner and tell them that the dog needed to come in for a test before we could send the script. The reason being is that if a dog is heartworm positive and starts preventative, it can cause a fatal reactioin. After a day or so delay, Petmed would start dozens (I'm not exaggerating) of phone calls, faxes, and other harassment of both our clinic and the owner saying that we were 'endangering the pet by holding up his/her medication' nevermind that by fulfilling the request THEY were putting the pet in more danger.
They are incredibly harassing and aggressive towards veterinarians and that's why most won't deal with them.
Back on topic, have you considered radioactive iodine for your cat? It's expensive up front, but is curative. I did that for my cat Kendall and it worked like a charm. No need for daily medication ever again!