I would never buy a drug treatment over the counter or without my vet advice, even flea treatments.
There have been many, many, many cases of cats (and dogs) becoming very ill and often dying from incorrectly applied flea treatments, wrong treatments, or just from a reaction. I came across several papers in my last job which means I don't take any risks with flea treatment.
Plus fleas can be resistant to active ingredients by area. My vet is best placed to know who is saying which drug isn't working locally, and they regularly rotate their treatments so I get one that works. Currently fleas here are resistant to frontline.
I want someone who knows my animal, has their records and other medications in front of them, and can sell me a safe, effective treatment. Going online or circumventing this process to save money is false economy at best, dangerous at worse. And that's from someone who has a degree and Phd in drug action and design.
Drugs are dangerous. They have effects, wanted and unwanted.