A bit of follow-up here. I still have my previous Android phone (on which I am able to make purchases) and happened to notice when I was in the accounts area of the Play Store that it says I'm connected to the US instance of Google Play.
(See motoG.jpg.)

However, when I navigate to that screen on my GrapheneOS phone, it says "Switch to the United States Play Store". (See grapheneOS.jpg.) There is no button or field to tap on, and tapping that text does nothing. So, something may be amiss with the sandboxed store? (At least, perhaps, for this particular installation.)