Did he display any unusual symptoms or behavior? Did he have any discoloration on his fins or body after he died? (Dark or gray areas or spots?)
Don't worry about going back to the petstore. I got one Betta simply because I saw a petstore employee handling him roughly during a water change, so I wanted to rescue him. I acclimated him extremely slowly, but he died anyway. When I went back to the petstore, they simply sent me to the fish section, where an employee said, "Do you want a refund or a replacement?" There were no questions as to what had happened. In fact, the employee sounded bored. I asked if this happens often. She shrugged and said, "Yeah. So, did you want a refund or a replacement?"
Of course, I took a replacement.