But that is a non-sequitur. Literally, in that that doesn't necessarily follow. We literally don't know why the buy box doesn't have it here and I could guess at a few other reasons. Most likely, I'd guess that the cheapest option not being Prime factors in to it more so than external offers being cheaper.

You're right, shipping speed likely also factors into it in addition to cost (that's why I also mentioned the shipping speed). If Amazon can't deliver within its 2-day Prime guarantee it is less likely to promote the offer.

Of course, without access to Amazon's systems I can't say for sure why any individual listing is or is not promoted. I'm just making an educated guess based on what the FTC put in its complaint.

Totally fair, and I think this is the kind of thing they could/should make rules around having a good audit mechanism for regulators to check.