Official policy = no
Purchases from the shop parks app can only be returned at WDW if they were delivered to you at a WDW resort. If purchases were delivered to your home they can only be returned by mail, and you pay the cost of shipping.
also
Returns and exchanges can only be made within 30 days of purchase.
Unofficially people have reported they have been granted exceptions to both of the above rules though, so it can't hurt to take your shirt and your packing slip (if one is included, that has been hit and miss for me on app purchases) and ask nicely, just don't be surprised if the answer is no.