I have opted back out of the KDP Select program for Sleight of Mind #1 so this is the last day it will be free in the Amazon store; probably ever. It ends up not making as much sense for short stories as it does for full-length books.
Sure, you get the free promotion days. On the other hand, I could just post a .mobi file on my blog and maybe get fifty or sixty percent of the value those provide and be able to do it for as long as I like to make up for the lower value of a given free day.
The real benefit of the KDP Select program is the fact that your book goes in to the Kindle Lending Library. That's a service wherein Amazon Prime members can borrow a book free for an unlimited period of time.
On the surface that seems great. Readers read for "free" - meaning they paid for the service but don't actually spend any money on your book - but you still get paid. Deeper analysis shows it to not be very valuable for short stories and here is why: You can borrow a book for as long as you want but you can only have one book borrowed at a time and you can only have one book borrowed at a time and you can only borrow one book per month.
Those two rules, mostly the last one, create a scarcity in the ability to borrow that makes it more like an alternative currency and less like a way to read for free. If you have a unit of currency and you are deciding how to convert it into a useful product, you're probably going to want to do it in the way that simulates the best "exchange rate" possible. That is, if you can only borrow one book a month, you're more likely to borrow a $9.99 book than a $0.99 book.
That means that, for people with expensive, book length assets there is probably some value to using the KDP Select program - a certain class of people, who are known to be heavy readers, can get your book for free and you get about half the royalties. The price you pay is that your book is only available on the Amazon store.
I've already gotten some borrowing activity on Madness & Loss and the royalties on that are comparable to what I would have gotten if someone bought it. That's fantastic. Freeish to the reader. Money for me.
On the other hand, if you have a ninety-nine cent asset like a short story, you get the ability to promote it for free - which you could ordinarily do yourself - and the ability to put it in the lending library - where nobody would want to waste a checkout on it. The price you pay is that you can't put it on any of the other digital services or send copies of it to any reviewers.
So that's why I'm having the Sleight of Mind series exit the KDP Select program. I may come up with my own "free for a certain period of time" plan when the 90 days is up and I will definitely start hitting up book review sites and other e-reader avenues. I just can't see a good reason to put a short story in the KDP Select program again. I can't imagine why anyone would.