I had a similar experience, but I checked back an hour after and ordered it with no issues.

I was really, really mad before that b/c I put it in my cart within 2 seconds of it going live, yet I couldn’t get past the shipping info page for 20 minutes, then while I was stuck at the payment page the error changed to “out of stock”.

Assuming you aren’t doing it for other work (as in non-gaming stuff), why do you need a PC upgrade if you are getting a steam deck?