Discount shipping cost based on number of items being purchased

wc-discount-shipping-qty-based.php

/**
 * Discount shipping cost based on number of items being purchased
 * Further narrowed down to only run on BE Table Rate options
 * Code snippets should be added to the functions.php file of your child theme
 *
 * @return array
 */
add_filter( 'woocommerce_package_rates', 'add_shipping_percentage_discount', 10, 2 );
function add_shipping_percentage_discount( $rates, $package ) {
	// get number of items
	$item_quantity = WC()->cart->get_cart_contents_count();

	// only run if multiple items ordered
	if( $item_quantity > 1 ) {

	    foreach( $rates as $key => $value ) {

	        if( $value->method_id == 'betrs_shipping' ) {
	        	if( $item_quantity == 2 ) {
	        		// calculate 20% discount
	        		$discount = $value->get_cost() * .20;
	        	} elseif( $item_quantity >= 3 ) {
	        		// calculate 25% discount
	        		$discount = $value->get_cost() * .25;
	        	}

	        	// subtract discount
	        	$rates[ $key ]->set_cost( $value->get_cost() - $discount );
	        }
	    }
	}
  
    return $rates;
}