Renewal price zero - still goes through to paypal

[castus +]

As it says in the title really. My client wants all ads to be free for a few months. This is fine when putting an ad on initially, but if someone tries to renew the ad, it still goes through to paypal, even though the price = 0

The code seems to have a payment provider in there regardless, so I think it might take more than just one or two lines to get this sorted. Any ideas or help would be much appreciated.

Thanks,
Gary