Additionally, the second Department of Temporal Investigations novel, Forgotten History, is on sale for some reason at $1.99. That's the only novel besides the Destiny and Errand of Fury trilogies that hasn't previously gone on sale. Amazon has also taken to randomly discounting grab-bag lists of novels something later in each month's first week, so I'll keep my eyes out for those.
One note of caution: if you're searching "Star Trek" in the Kindle Store, sorting Price: Low to High, and clicking on "All Results," there's something called "Star Trek: Lost Frontier" by an Eric Busby that is using the No Limits anthology's cover as its artwork. It was posted on July 14th, 2018, and clearly isn't legitimate. Don't give that guy money by mistake!