ec_charges e-commerce

Sorry if this is the wrong place for this discrussion -

I am struggling with the charges module. I gather from this post http://groups.drupal.org/node/21247 that I am looking at the right part of the ecommerce admin pages, but I can't make any sense of it.

Could someone point me to some documentation (or which files in the module I could study to figure out how to use it)?

I am trying to use a CCK field to add a postage cost. I would like this cost to be added on to the price of the product if the field is not empty.