You would like to pass on your credit card processing fees to your constituents. You want to add that fee into the price, if a registrant chooses to use a credit card or PayPal processing (instead of paying by check).
Organizations can use the Credit Card Surcharge option found under Tools -> Payment Settings to cover the cost of the transaction fees associated with credit card processing. To add a Credit Card Surcharge, which will be applied for PayPal or Credit Card Transactions only, complete the following fields: Credit Card Surcharge Name and Credit Card Surcharge Rate. You may select a surcharge rate of 1-15%. As an example, PayPal users may charge an additional 3% of the transaction to cover PayPal's transaction fees.
If you wish to add a tax or surcharge to all transactions regardless of payment type (ie including checks and cash), please see:How can I add a tax or surcharge to transactions?