returner's remorse
The regret one feels after returning an item they purchased or received as a gift.
Rick felt returner's remorse shortly after returning the new laptop he purchased 30 minutes earlier.
remorse return
A "remorse return" is returning something to a store when you feel terrible about having blown the cash on the purchase.
Mack was so sick about blowing 3 grand on a kayak at REI that he took it back and got a remorse return.