I have modified the table.php shipping module to calculate shipping costs based a percentage of product price for each category. Now I need one more thing... I want to add a checkbox (on the estimator page as well as the checkout shipping page) that can be checked if the buyer has a residential shipping address. Then my code will add a predetermined surcharge for the residential address.

I am using the Table Rate module so I don't need any UPS or FedEx stuff, just a simple checkbox so they can click it if they want to signify that we are shipping to a residential address.

I imagine I need to set a global variable (ie: RESIDENTIAL_ADDRESS) that will store if they choose the residential address checkbox either from the shipping estimator screen or the checkout shipping screen and then my code in table.php can check that and add the appropriate amount to the total shipping cost variable ($this->quotes: 'cost' => $shipping).

Now I'm foggy on how to actually do this though. I know I'd edit the template files to add the checkbox but how do I actually track/store the varaiable?

Thanks for the help!