Hello,
I am senior Prestashop developer working with Prestashop more than 6 years (installation, core functionality & template modification).
As I have done similar task (using second product price /custom attribute/ in cart price calculation when totoal product weight in the cart weight exceedes 10 kg) one week ago, I am skilled enought to narrow out-of-the-box Prestashop functionality according to your requirements.
Are you interested? Please contact me with more details. I can start working immediatelly.
Brano
Lead Developer | esentra