Oh hey, it turns out this actually is a deliberate promo from the MZB Literary Works Trust,
according to their website.
This "holiday gift" will only be available through the 27th, which means that if you missed it and you want it, you can still get it for the next 8 hours or so until the Amazon servers switch over to "midnight" Pacific time.
And the 55% off coupon you can use on MZB's Fictionwise books will still be good through tomorrow, most likely.
Avoid getting her individual Darkover short stories there. They're nearly all collected in her
Marion Zimmer Bradley's Darkover collection (which reprints the old DAW paperback anthology, and is a very good intro to the setting, featuring some of her best writing work) for a much lower price, and there's only one of them "missing" out of the lot, which you should be able to easily figure out once you see the table of contents in the ebook.
Same caveat with her other sf/fantasy short stories, many of which are collected inside anthology bundles.
ETA: Also, bonus KDP Selectively-freebied short story by Elisabeth Waters, manager of the MZBLWT:
Truth in the Inward Parts, part of her "Treasures" series of shorts available on FW, and should be free across all the Amazons.