You are working with eCart correct? If so you can just edit your current cart object and in the shipping tab you can setup a new shipping charge rule. In the rule you will set the trigger up with two conditions. The first condition will be to check if the session variable exists, then the next condition will be based on the value of that session variable, if the session variable is > 0. Finally for the amount you can just specify a flat rate then for the value enter your session variable.