What percent does American express charge a retailer for using a credit card?

Typically 2-3% for MasterCard and visa. I believe American Express is more.

Texas Parks and Wildlife charges 2.25% extra to renew your boat registration online. I think that is to make up the credit card charge backs. ( on a $53 fee).

I think creditors also have a minimum charge so that on small purchases ( < $1-4?) the retailer looses money when you use a credit card.