In our * 40 Year Reunion planner link, I created an 'item for sale'

http://www.classcreator.com/Freeport-TX-Brazosport-1970/member_ReunionSignup.cfm

$5.00 for each item [which is simply a monetary donation]to help out on expenses and the next reunion

One classmate bought 20, and it shows up as $100.00
Another classmate bought 20, and it showed up with the 'early payment discount' - total being $95.10

What have I done wrong?

Click on your main Reunion Planner link again, then go back into edit mode. You have this turned on:

Give Classmates a 4.9% discount when using payment type other than credit cards.

If you don't want the 4.9% discount turned on when paying with payment types other than credit card you should turn that off. Here one person paid with credit card, the other paid with a check. Because you have the discount turned on, the person who paid with the check paid slightly less.