Something somewhat-similar happened to me on launch day. I had preordered at Amazon and Best Buy. Amazon preorder was successful, so I canceled Best Buy. I got an email a couple days before launch from BB saying that my Breath of the Wild Pre-Order was successful and that it'd be available for in-store pickup. I guess I purchased it in a haste without logging into my account, because there was nothing in my account indicating a Zelda Pre-Order (I do remember ordering it though).
I walk into Best Buy that night just to cancel the order so I wouldn't get charged. I go to the register, tell them I want to cancel. They ask my name, find the box, take the sticker off, and say that they'll cancel it. I walked out and got a cancellation email a few mintues later.
They never asked for ID. They never asked for a receipt. They never asked for any proof that I was who I said I was. This was a manager who did it, too. Some stores just do not give a shit about policy.
Everything worked out for me, but it was extremely odd to me that they didn't ask for a shred of proof.