you are not using the correct triggers.
your triggers are:
Subtotal of Column quantity > 0
AND
Subtotal of Column quantity <= 2
you need to use the triggers:
Based on USPS Shipping quote value > 0
AND
Subtotal of Column quantity <= 2
the first trigger needs to be:
Based on USPS Shipping quote value > 0
not:
Subtotal of Column quantity > 0