Followup - I did some checking as my kdp book does not have an author-buy-at-cost option. Here's amazon's reply to my reaching out to them:
"Unfortunately, the "Author copy" feature is currently available only for selected publishers and those publishers are selected randomly. As you know, we are still working on this "Author copy" implementation for all KDP publishers. This is now not made available for all our publishers as there may be many practical difficulties which we may not be aware of until implementation, since it is a beta version and we will rectify all those practical difficulties faced by these selected publishers (for this feature) and launch the feature completely (for all KDP users) without any major issues on this. This way, all our publishers will not have to face all these difficulties on the initial phase of launching this feature.
Rest assured that this feature will be made available for all our authors/publishers using KDP and since it is now available for beta publishers, this information is being posted on our help page. I'm sorry for any disappointment.
I will escalate your concern to our business team though, as they may consider sending separate notification ( regarding beta launch versions ) only for selected publishers who can use this feature instead of posting this on our "Help" page, which will certainly avoid confusion to other publishers/authors those who are unable to use those features until it is completed launched."