Adding Weight Value to Products for Paypal Integration

I am using MarketPress along with a standard Paypal Express Checkout eocommerce solution. I'm trying to determine delivery by weight and zip code, but MarketPress does not feature that functionality. I can add costs per weight in Paypal itself, but have no way of adding that value in the plugin. Please help. Shipping has got to be the most confusing part of all this. Thanks!