cost of payment

I've searched but I haven't found a problem like mine. Sorry if I missed the answer.
I have a problem: I would like to add a cost (for example 5 euros) when a customer choose the cash on delivery payment method. That is to say that if he uses paypal there is no extra cost, but with cash on delivery the extra cost is present.
Should I modify the cod.php file? Any hints for solving this problem?

Thanks a lot

PS: my opencart version is 0.7.9RC4

Add a shipping charge?

hm2k wrote: Add a shipping charge?

Yes, it can be a great idea! So I should add the cost to the previously calculated shipping cost? Any idea on how to access these data inside the PaymentCod (extends Payment) class?
Thanks

Actually I have a contrib out there already called "Calculate Surcharge" that adds a surcharge based on weight min/max and price min/max. But I was thinking about adding surcharges for payment and shipping choices as well.

I'll take a look at this today