Add cost to payment method in Marketpress

Hello, I'm trying to create a custom Cash On Delivery payment method in Marketpress.
I have cloned and renamed the manual-payments.php file, edited it's contents just enough to rename everything and now my issue is this: The Greek Postal Service offers the option of cash on delivery but with an additional cost of €2.3.
So when someone chooses COD as their payment method, €2.3 mush be added to his shipping costs.

I have tried adding this:

$eltaprice = 2.3;
$shipping_price = $shipping_price + $eltaprice;

before line 113 and also tried some other variations but the shipping cost doesn't change no matter what I try.

Has anyone tried anything similar? Any tips?