Well, it's not necessarily "less work," I mean- in a way it is.
Diana's the only one able to do that currently, so I can't go in and do it for you. :/ And she's always really busy.
BUT, even if we could, since you did "purchase" them, they are technically just doubles. So when trading is implemented, you'll be able to sell the things you accidentally bought. Although I know that doesn't currently help for the Halloween event, but I promise we'll be keeping these items in the shops long enough for people to get all of them.
Again, I'm really sorry! The pop-up is there to prevent that sort of situation from happening, so I always leave mine unchecked when it asks the "do you never want to see this again?" thing, haha.