Here is the link to the 2009 MasterCard Chargeback Guide. It states in rule 5.9.3 that a merchant may not limit the maximum transaction amount. Here is the link to the document:
http://www.mastercard.com/us/merchan...nual_10_08.pdf
If you are going to try this, you may as well be armed. I expect thes documents are available for VISA and Amex as well. Happy shopping! toni