They don't seem to be regular FC employees, but I also don't think they're bots. Instead, they appear to be humans with a limited understanding of english who are required to reply based on a manual (like call-center workers).

This explains why different accounts swap out -- the manual was designed for a single conversational thread, but employees are supposed to reply to individual tweets (as picked up from some kind of filtering system).

This is a sort of common pattern -- basically whenever I mention a brand on twitter, I get replies from somebody with "<brand> ambassador" in their display name, & those accounts never follow anybody or reply to their replies. It's gotta be a whole industry. The only difference is that, unlike the accounts run by Comcast or TD Bank or Steak-Umms, amazon's manual seems to demand that the service workers pretend to be regular warehouse workers (rather than customer service / PR).