I wonder how many of these are intentionally randomizing the form you get during checkout, in an effort to thwart automated purchases by bot accounts. I don't know anything about this space, but I've definitely noticed that my bank does this with its login flow. There's at least a dozen variations and domains I might get redirected to on any given day. Does anyone who works in this space know of a similar practice for online checkout?

Really??? That sounds insanely insecure by design. If I tried to log in to a secure site and was redirected to a random site I'd assume I was the victim of a link backing attack and back out. Normalising this kind of behavior by a bank sounds like a very bad idea.