Fix when using thickness depending shipping modules

If you are using one of the thickness depending shipping modules:

Thickness depending letter only shipping module
http://www.oscommerce.com/community/contributions,4493
or
Thickness depending letter or package shipping module
http://www.oscommerce.com/community/contributions,4484

you have to make small change to estimated_shipping_functions.php, so the shipping modules can calculate the correct shipping costs.

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