Unicentaopos 3.81 Credit card Surcharge button

Hi Coders, 1. I'm looking for someone familiar with Unicentaopos 3.81. I need to have a credit card surcharge button in our system. For example 2% impose on the total amount when payment is with credit card. Cashier do not need to key in 2%; but simply click on the Surcharge button. I did manage to set it but the script won't allow me to make payment because codes dictate I have to send all items to printer first. 2. To round off the amount to 10 cents.