the issue you have is that if the value of the shipping calculation is 0, then nothing will appear in the shipping line of the cart
what i would be tempted to do is change the shipping calculation, so that it is a fixed value.
this will obviously be set regardless of the session value selected, but at least it will show that the session variable is being set and detected.
once this works, change the calculation
if all else fails, zip and post the offending pages