Trying to optimize free shipping module

Hello.
I want to give free shipping if the costumer pays by bank transfer over 150€ (that I have managed with the free shipping module), the problem is that if the costumer choose the option cash on delivery I want him to pay a fee of 3€. The shipping is still free but he has to pay for the cash on delivery expenses.
Is it possible?
How can I do that? By changing the free shipping module? Or by duplicating it and made some changes? I've spent hours looking for the code but don't know how to do it.

Thanks in advance
PauloDuarte