ho trovato la risposta nel forum di awocoupon
For example, in the shipping plugin weight_countries, in function getCosts, this is the logic to determine free shipping:
<?php
if ($method->free_shipment && $cart_prices['salesPrice'] >= $method->free_shipment) {
?>
This does not take into account coupons, I would change it to the below, (code borrowed from payment plugin standard)
<?php
$amount = $this->getCartAmount($cart_prices);
if ($method->free_shipment && $amount >= $method->free_shipment) {
?>
il file da modificare si trova www/plugins/vmshipment/[shipping_method]/[shipping_method].php