Good news and bad news.
Good news: I was able to get past this error message by removing the /US from the URL. So anyone having this problem - worth a try.
Bad news: Now I'm getting the error message below. When I click "Select a location," every single store says "Unavailable." Unable to checkout at all now. I have extraordinarily low expectations of fixing this. Looks Like I may be giving Apple a call later today, if I can even get through.