Shipping Charges by Zone

I have a client with a two tier shipping charge system (according to Zip Code) already in place with another cart solution (not WA). He would like to change to PS3, but maintain his current shipping charge scheme. So, there will be no real time charges from any of the shipping companies supported by PS3, but instead a scheme where a shipping charge will be added according to the "zone" where the buyer lives. There are just two zones, and every product has a zone 1 ("west of the Mississippi") and a zone 2 ("east of the Mississippi" shipping cost associated with it.

Can this scheme be integrated into PS3 in place of the built in shipping solutions? I would expect to have to use eCart5 which I own.

If it can be done, can you please give me a rough outline of the steps needed? To begin, I know I will have to add the two fields (zone 1 and zone2) for the shipping costs, either added to the ps3_products table, or create a new three column table (ps3_shipping) with fields for ProductID, Zone1, and Zone2.

After that, please list the PS3 site pages that would be affected by this kind of change (besides shipping.php).

As an aside, would the DreamweaverCS5 feature that integrates CMS be of any help with "seeing" the pages involved in this makeover to PS3?

Thanks,
Robert