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.
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.