458628 Project Urgent - Editing Virtuemart payment module

Project Urgent I use virtuemart on joomla 1.5.20 and v1.015 module Paybox payment (attached) is the only payment module that I use. the concern is that I paid products and free product in the shop. I have about a product paying the paypal module works very well. but as I have a free product to zero the paypal module also affects itself automatically in the mode of payment of the proceeds! I wish I were altering the module attached to: - If the product is paying it does not change - If the product is 0 do not activate activate paybox is nothing like payment module thank you