Because if you stick to your list, it’s affordable. [Just don't wander around to "see what else they have."]
And because a couple of times, they have had the good taste to carry my books. I cannot tell you how exciting it was to see them there. I pointed them out to anyone within twenty feet of me. I've even done a signing there. You sell a ton of books, but most folks are not used to seeing someone who isn't sampling.
I’ve got my fingers crossed for my next adult book. If you would like to read about how Costco chooses which books to promote, click here – it’s an article in their magazine. “It’s not uncommon for Costco to purchase 20 to 25 percent of a new book’s first printing.” While I know I get less royalties for these books (because they are considered “deep discounters”), I also take advantage by buying copies myself. I can get them for cheaper than my author discount, and I still get some kind of royalty on them.