Personally I try to avoid Melafix entirely on my bettas... There's been a lot of research showing that even small doses of the medicine can damage their labyrinth organ. Bettafix is a better one to use on them, along with raising the water temp to at least 82 and adding a bit of salt.
Hard to tell from the pics if it's fin rot or if the betta's been munching on his fins a little. Sometimes bettas get bored while in the tiny cups and will chew on their own tails to relieve boredom. The raised temp, salt, and Bettafix will help with that too. Another possibility is that the betta has a touch of poorly bred crowntail in him, though I think that's less likely. Anything is possible with a Petsmart betta though, as they don't exactly have breeder stock.