Custom integration of shipping/tax with paypal

I would like to apply my custom shipping and tax rates, which are dependent on the user’s zip code and items purchased, within Paypal’s existing checkout experience. I believe this will involve using Paypal’s express checkout product; I’d like to use php and mysql to do this. There are numerous sample scripts on the web but I’m unclear on how to integrate them all; this is what I’m looking for in the solution. Once the user has picked out products for their shopping cart and has chosen to check-out, they are prompted by Paypal to enter their shipping address. Based on this address (zip code specifically) I would like to return the correct shipping and tax rates. For shipping, I have custom rules I’d like to enable. I have a basic understanding of PHP and would like to add my custom rules based on the initial rules set. For any products with attributes of Old Navy, Banana Republic, or GAP the following shipping rules exist: • If the total from any products with these attributes is greater than $50, the options listed should be 7-9 days ($0), 3-5 days ($7), and 2 days ($17), and 1 day ($22). • If the total from any products with these attributes is less than $50, the options listed should be 3-5 days ($7), and 2 days ($17), and 1 day ($22). For other stores, I would like the ability to enter other shipping rates. If for example there is a product with attribute Best Buy, the total shipping should be whatever shipping I set for Best Buy. If a user selects a combination of items with attributes of Best Buy AND Old Navy/Banana/GAP, the shipping rules should combine (shipping for Best Buy + shipping for Old Navy/Banana/GAP). The similar logic should apply for however many retailers I add. I will provide the values for each of the attributes. Tax should be applied to the total amount of the products in the cart and the total shipping rates. Tax rates are already set based on a table of zip codes, which I already have and are stored in a mysql database. I will provide the table that lists tax rates by zip code. Notes: I’d like to have the code commented so it’s easy to follow in the event I’ll have to make revisions/additions in the future. It would be great if you could write the general script with the paypal integration and I will input my paypal account information to verify the functionality.