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We use Mal's Ecommerce to allow people to pay us via credit card. They are supplied with a link to a page with Mal's form script on it that allows them to enter the amount on their invoice and submit for payment.
We would like to have three mandatory checkboxes that calculate values based on which card they are using. Eg, VISA and Mastercard, 2.4%, Amex, 3.5%, Diner 2.5%. The check-out value will be the value they entered as their invoice amount multiplied by the percentage that corresponds to which credit card they chose.