Woocommerce - modify shipping cost depending on quantity

woocommerce-modify-shipping-cost.php

// Free retail shipping
add_filter('woocommerce_package_rates','ff_override_flat_rate_price',100,2);
function ff_override_flat_rate_price($rates,$package) {
	// Modify shipping cost for Books depending on quantity
	if( ff_get_cart_total_quantity() >= 2 && ff_get_cart_total_quantity() <= 3 ) {
		// 2 or 3 = 15.95
		foreach ($rates as $rate) {
			//Set the price
			$rate->cost = 15.95;
			//Set the TAX
			$rate->taxes[1] = 0;
		}
	} else if ( ff_get_cart_total_quantity() >= 4 ) {
		// 4 or more = 19.95
		foreach ($rates as $rate) {
			//Set the price
			$rate->cost = 19.95;
			//Set the TAX
			$rate->taxes[1] = 0;
		}
	}

   return $rates;
}

function ff_get_cart_total_quantity(){
	global $woocommerce;
	$quantity = 0;
	foreach( $woocommerce->cart->cart_contents as $p ) {
		if( $p['data']->virtual != 'yes' ) {
			$quantity += $p['quantity'];
		}
	}
	return $quantity;
}