Q:  Can I require a minimum purchase or charge a fee to accept a credit card?

No.  I know what you're thinking, "But I see this all the time when I visit stores."  I do, too, but this is completely against the Visa and MasterCard agreement you signed when you were setting up a merchant account.  The card associations feel it is discriminating.  If caught, your merchant account company could drop you for breaking the contract.   According to the sites I mention below, it's very cut and dry... the guidelines are you must 1) not charge a minimum purchase amount 2) not charge a fee to take a credit card.

On the Visa site below, scroll down and click on: "Minimum Purchase"

http://usa.visa.com/about_visa/ask_visa/index.html

Here's an article on Bankrate.com regarding this: 

http://www.bankrate.com/brm/news/cc/20010418a.asp?prodtype=cc

Company Suggestions

Recommended Resources:

Merchant Accounts to accept credit cards - MerchantSeek.com - Eliminates the hassle of searching the internet for credit card processors by providing a one stop resource for merchants looking for credit card processing information.