add the checkbox to the checkout page named installments and set the checked value to 1:
[php]<input name="installments" type="checkbox" id="installments" value="1" />[php]
on the confirm page, there is a collection of hidden form elements near the bottom, add a new one with the same name as the checkbox and set to capture the value posted from the checkout page:
<input name="installments" type="hidden" id="installments" value="<?php echo((isset($_POST["installments"]))?$_POST["installments"]:"") ?>" />
open the server behaviors panel on the confirm page and click the plus button, select eCart -> General -> Set Session Value
for the trigger and the value click the lightning bolt icon and select the installments form element, set the name to "installments"
in the eCart object, create a new charge.
Name:
Installment charge
Triggers:
"Bassed on Session Variable Value"
If the session variable "installments" has a value of "1"
Calculation:
"Based on cart subtotal"
cart Subtotal "times" "0.1"