Custom Shipping Module for Zen Cart

I need a custom shipping module for a zen cart site. Need to do the following: Different item carries different flat shipping rates to all locations in the US, and a surchage for Canada customers. I would also need some type of combined shipping option, first item at a fix rate and then addition units of that same item will cost less... For example: Item A would cost a flat $10 to ship to anywhere in the US, with a $5 additional surcharge for Canada shipping. Second unit will add an addition $3 for shipping charge. Item B would have different rates but same type of setup. I need the rates to be customizable, they will not depend on size or weight, just whatever rates we feel appropriate.