I actually did a spreadsheet back in June and laid out the percentages of these companies: Schoolpop, SchoolCash, iGive, uPromise, Boxtops and SchoolShare. I'll e-mail it to you, but keep in mind it's a few months old, so you might want to doublecheck it--some percentages may have changed. (If anyone else wants it--I'll send it, but please e-mail me directly; don't ask within the text of this thread.)
We ended up promoting iGive to at least start out the school year, because you can make a lot of money through their referral program (which is scheduled to end on 12/31). After we get people used to the concept, then we'll probably ask them to switch to Boxtops or SchoolCash (or at least to compare the two). iGive has a much broader shopping mall than the other companies, but the percentages are much lower.
Someone else (I think it was PTA Renegade) was also looking into settting up individual merchants' affiliate programs on her school's website, because you do get a highter percentage that way (see
www.cj.com
-- pay for performance section) . We just don't have the volunteers to be able to work on our website for us to get that going or maintain it. But if you do, it might be another viable option for you.