Configuring Paypal 2.4 Module - Where To Change Variables?

[Yun]

Hi There,
I am trying to configure the Paypal 2.4 module for Website Standard Payment on a fairly non-standard setup of Prestashop version 1.3.1.

With Google Checkout, I am able to pass the details of the item description, price and weight and allow GC to calculate the rest. I would like to do something similar with Paypal, so that it calculates tax and shipping based on the Paypal account shipping address.

Through my research on the Paypal developer site and through their documentation, I know that there are variables that I can use to prompt a customer to supply a shipping address. I have also set up my Sandbox account so that values passed from the cart do not override the tax and shipping calculation tables I have set up.

So - the account is set up according to the book, but no matter what variables I put into the redirect.tpl file, they make no difference to the information populating the landing page which shows only the cost of the item without shipping or tax or any of the details I used to pre-populate the variables.

Am I changing the variables in the wrong place? Where should I be inserting the variables to change my customer's checkout experience?

I've been grinding away at this for a long time now, and any help would be appreciated. Things are starting to drive me a little crazy!

Paypal settings can be seen in attached images.

Thanks!
Iain