Add code to oscommerce contribution

Hi We have successfully installed a shipping contribution, to allow us to offer free shipping if customers purchase from certain categories, however we would also like to limit the free shipping to only if they spend £150 or more so we needit modified to only allow free shipping after they reach £150. Should be very easy and quick so bid low and we will add you to our provider list and you are guaranteed more work from us. We are an agency. Many Thanks