Set the Shipping Cost Method
function swwp_wc_shipping_cost_tiers( $cost, $method ) {
if ( in_array( $method->get_instance_id(), array( 1, 23 ) ) && WC()->cart ) {
$cart_item_count = WC()->cart->get_cart_contents_count();
if ( $cart_item_count > 3 ) {
$cost = $cost+ ( 1.2 * ($cart_item_count-3) );
}
}
return $cost;
}
add_filter( 'woocommerce_shipping_rate_cost', 'swwp_wc_shipping_cost_tiers', 10, 2 );