FedEx Shipping Calculator

Ok, so I’m coding a shopping cart.

I already setup the system to add items to the cart using SESSIONS and getting your total. Now I need to add the shipping costs.

I want the shopping cart to calculate the shipping costs and add it to the total. Looks like FedEx has several Web API’s to do this but I’m finding it difficult to understand.
http://www.fedex.com/us/developer/

Anybody knows about an easier way to calculate shipping or can point me as to what I need to know or study?