Given how busy some of us are, it can still be quite easy to miss that Steam Deck email coming in to make your purchase through the reservation system. The good news is, you do actually have a few extra days after the timer is up.
The way the system works is that you put down a reservation (only one per Steam account at a time), and eventually you get the chance to buy a Steam Deck. This has helped against scalpers buying them all up and allow Valve to have time to produce enough. Once you get your final purchase email, you have about 72 hours to actually pay for it otherwise it's supposed to go onto the next person in the queue. The purchase email even clearly says " If you don't complete your purchase by this time, we will automatically cancel your reservation.".
However, Valve has now officially confirmed that's not quite the case.
If you do miss that window of opportunity to buy your Steam Deck, there "is a grace period of a few days" where you can reach out to Steam Support who will hook you up. All you need to do is go to the support page, and hit "Contact Steam Support".
Nice to see Valve are giving people more than one opportunity to ensure they get their Deck.