Add percentage to subtotal when selected option is

So I’m using a payment provider wich includes all the gateways (iDeal, paypal, creditcard etcetc), it’s called Multisafepay.

However, when customers buy a product using a creditcard (Visa), the store owners pays a fee of 2,9%. The store owner wants to add this to the subtotal of the cart.

I know how to do this by changing the core-files (cart.class.php if I remember right), but I want to make a filter for that to use it in my themes functions.php, how do I do that?

Second question: the subtotal must be updated when the Visa Card radiobutton is checked. Anyway how to do that?

Thanks very much!

https://wordpress.org/plugins/wp-e-commerce/