If you would like to add a credit card surcharge to your bookings for guests paying by credit cards you can add an extra that charges by percentage.
Go to the Settings > Extras while in Preno.
Enter your Extra name e.g. "Credit Card Surcharge".
Enter the percentage of your surcharge.
Tick "Charge by percentage".
When the Credit Card Surcharge extra is added to the booking, it will add an extra which is a percentage of the booking total.