When you get a new fish, really, in spite of the best care you give, sometimes it happens. Might have been sick already. You just can't tell what kind of care they got prior to coming home with you
Most places will give a refund or new fish for up to a month if they do have problems after going home.
I had a perfectly healthy and hearty looking red veil die after three days...you are SO not alone in what happened. I'd imagine most of us on this forum have experienced it at least once.
If you decide to try again (hopefully you will!), you might keep the new fish in a smaller quarantine tank for a week and maybe gradually introduce water from your cycled 10 gallon to it? That way you can make sure he's healthy and won't cause problems for anything else in the big tank, and you can give him a longer acclimitization time.
I'm sorry for your loss - these dancing fish really hook you fast.