According to one of the employees of Epic Games, in patch 9.10 we will get a cancellation function. At the moment, it is not known exactly how this function will work, but most likely, after the purchase we will see a cancel button.
It is worth noting that this problem has long tormented Fortnite players. Buying a skin is done in one click, which can easily turn out to be random, and the old return system was inconvenient not only in that it was limited to 3 return requests, but also returned money for the last 30 days.
Have you made random fortnite purchases?