I have looked at the code on your page and it looks like it has been configured to work with the weight for each item that you have in the cart. What is the result that you are getting with the multiple items?
If you would like to add in shipping from PayPal you can configure this in your profile. You can set up flat rates or based on a few options. Here is a link to the page with more info:
For adding FedEx you just need to obtain an account and meter number, you should be able to get this from the FedEx website. Adding this in after the fact would require you to check on the charge rule to ensure that it is listed and you would also need to update the cart display on your confirm page. If you are going to use FedEx it might be easier to backup what you have now and just run the checkout wizard again.