Most donations at iGive are based upon a percentage of the purchase price you paid. However, some donations are figured in a way too complex to express as a simple percentage. Perhaps they're a flat rate donation. For example: "$1 for flights; $3 for hotel bookings" Or perhaps there is a cap or maximum donation amount. For example: "$.80 per ticket, maximum."
These are listed as "Special Rate Donations" and full details can be found on the merchant's landing page as and Exception on iGive (or in the lower right hand triangle if you reach the site via the iGive Button).