Petco and Petsmart both have 30 days return policies on ALL fish. If a fish dies they take it back no questions asked and give you an exchange or a refund. We have brought home many, many bettas who were sick and on the verge of death to try and save them that didn't make it. They have never refused to do a refund, it's company policy and whoever told you that they didn't offer refunds on bettas were not telling the truth.
I've also gotten a lot of discounts for the sick fish, although we have paid full price for most of them.