I wouldn't bother with it. It looks to me that if you sell less than 2000 books, the *most* you can get from the sale is 10%. The percentage could work out to a little higher if you sell more than 2000, but depending on your population, that may be completely unrealistic.
I just don't see the attraction on this. They may think that the 5% discount will get you sales, but as a PTA member I can get a 5% discount on books at bn.com everyday. Or I can go to Amazon and buy for less, plus have a percentage of my sale go to charity if I go through a charity's website. But I'm also a big online shopping fan. If my kids had required books, which is what this seems to be geared towards, that's how I'd buy them.