This is the original poster returning with a further issue. For some reason the web site required me to "complete registration," allowing me to define a real user name, although it never gave me a chance to do that before.
(When I logged in, the site told me that my account was blocked due to activity that violated a user agreement or something like that. It forced me to enter the same captcha twice to unblock me. I am concerned about this; the unauthorized activity definitely wasn't mine.)
Now, on to the further issue. I contacted Recipe Keeper to request assistance with the problem. They never replied. Shame on them; I can't use their app if I can't buy it, and if this is the level of user support they offer, I wouldn't want to. I'd be interested in pursuing a complaint against them as a Microsoft Store seller, if such a thing is possible.
However, today I'm trying to buy another app (Lifetime.AnyScan.IAP, a/k/a Doc Scan PDF Scanner), and the same darned thing is happening. The "buy" dialog has a Next button which opens a second dialog to confirm my address, and the second dialog has a Back button that returns to the first dialog, and there's no way to break out of the loop.
So the same problem appears when I try to buy two apps from two completely different sellers. This is not a problem with the seller's software. Can Microsoft help me fix it? No one else is going to.