Similar thing happened to me. Short version is I bought the 256GB deck for my husband only to disappoint him by not getting the 512GB version, so I contacted support to upgrade or cancel before it got shipped out. They refunded it, I purchased the big boy, and both decks ended up getting shipped and delivered, with no record of the first purchase/refund on my account even after contacting support, and I guess it would have been more trouble than it would have been worth for them to get it returned, so we got lucky and now husband and I have a pair of his-and-hers Steam Decks for the price of one.
Similar thing happened to me. Short version is I bought the 256GB deck for my husband only to disappoint him by not getting the 512GB version, so I contacted support to upgrade or cancel before it got shipped out. They refunded it, I purchased the big boy, and both decks ended up getting shipped and delivered, with no record of the first purchase/refund on my account even after contacting support, and I guess it would have been more trouble than it would have been worth for them to get it returned, so we got lucky and now husband and I have a pair of his-and-hers Steam Decks for the price of one.