Quick PSA: Lots of SD Cards are heavily discounted on Amazon Prime for Prime Day.
I haven't gone through and done any specific inquiries about which cards are suitable for the SteamDeck, but I figured I'd bring it up for anyone looking to upgrade their SD card storage.
Counterfeit SD cards (of all brands) are extremely common on Amazon. If you purchase one, be sure to thoroughly test that its capacity and speed are as advertised.
I got an obvious fake Samsung card which didn’t even work. When you try to leave a negative review, Amazon tries to gaslight you and tell you that the card isn’t counterfeit.