dont edit the code, use the eCart object editor to modify the shipping rule.
open the confirm page or any other page that has eCart applied to it, then open the server behaviors panel (Window -> Server behaviors).
in the list of server behaviors, double click the eCart object server behavior.
in the eCart object editor, go to the shipping tab and edit the USPS Shipping rule.
add a new trigger to the rule:
"Subtotal For Any Column"
If the subtotal of column "Quantity" is "<=" a value of "2"