How can I add an extra charge in the invoice of 'Payment service cost'?

[Agusfn]

I am using a payment processor that has a cost of using it, a percentage of the total ammount. So i want to make it so when paying with that method, an additional fee is added to the subtotal of the invoice. How is this possible through a module?

I've learnt the basic information about developing modules, but I couldn't find info about adding a new "charge" to the invoice and altering the purchase process. Or is that made through editing the core?

 

Thanks in advance.