wc-tiered-shipping-costs.php

<?php // only copy if needed!

/**
 * Filters the shipping method cost to account for tiered quantity pricing.
 * 
 * @param float $cost the shipping method cost
 * @param \WC_Shipping_Rate $method the shipping method
 * @return float cost
 */
function swwp_wc_shipping_cost_tiers( $cost, $method ) {

	// TODO: change the numbers in this array with your desired instance IDs
	// see if this shipping instance is one we want to modify cost for
	if ( in_array( $method->get_instance_id(), array( 22, 23 ) ) && WC()->cart ) {

		$cart_item_count = WC()->cart->get_cart_contents_count();

		// if we have items that need shipping, round the quantity / 2 to the nearest whole number
		// this produces tiered cost increases for every 2 items
		if ( $cart_item_count > 1 ) {
			$cost = round( $cart_item_count / 2 ) * $cost;
		}
	}

	return $cost;
}
add_filter( 'woocommerce_shipping_rate_cost', 'swwp_wc_shipping_cost_tiers', 10, 2 );