The Calculation should be added to the eCart object.
Go to Insert -> WebAssist -> eCart -> eCart object to open the eCart object floater.
in the eCart Object floater, select your eCart object and click the edit button. create the calculation on the calculations tab.
NOTE: the calculation above is an example, the example uses a column named Color. If your column uses a different name, you will need to adjust the example code.
If you want to use a checkbox, you will need to edit the code that is produced by the add to cart button behavior. in the add to cart button behavior, set the column to be updatable from a textbox, then in code view, change the type attribute:
to be a checkbox