Buy one get one free on the Series S is an amazing deal, but I'd argue it's not too good to be true nor an obvious mistake. These systems have been in stock for ages, and we even heard reports a few months back that they were not selling anywhere near as well as expected. Microsoft could very easily have agreed to offset the cost of this deal if it meant getting more Xbox systems into households this xmas before the PS5 gets back in stock.
Secondly, Gamestop had every right to "cancel" the deal and orders - but if they were too slow to react and the consoles are being shipped, then they need to either intercept or honour the deal. I'm 99.9% sure it's illegal to increase the cost of an order after the transaction has completed and the order is already being delivered, technically they can do it due to the way they charge cards, but the only way they could do this that would hold up in court is if they are confident users DID get shown the correct price, at least somewhere, before placing the order. i.e. if the receipt says the correct price. If that isn't the case then they are fucked and congrats on your second underpowered console.
We've not seen a single person show a picture of these orders yet, nor has a payment actually cleared for the additional amount - so who knows what's going to happen. My guess is a minority of users get both, most get one and a voucher saying sorry.
It was never listed that you get an XSS for free or heavily discounted. The folks adding a second XSS to their orders KNEW that this was a listing error because the actual promo isn't for the XSS.