Warm, clean water is the best medicine for fin rot. AQ salt is good as well or stress coat. AQ salt can be used for I think up to 10 days. Make sure to dissolve completely in separate water before putting into your betta's tank.
The thing with pet stores, particularly chains is that if they give you the right stuff and the right information they probably won't make money. For example, the filters you buy come with those blue pads. The box says to change them every two weeks or something. You're not really supposed to change them at all and if you do you're likely to throw your tank cycle off. So it's not entirely the pet store's fault, it's just money and business. Unfortunately whereas we see a live animal who deserves a good life most stores only see $$$.
Best of luck ^^ Keep us all updated on your progress!