WooCommerce Table Rate Shipping Builder

<?php
/**
 * Table Rate Shipping: a tiered ("table rate") shipping method, tiered by total weight.
 * Registers a real WC_Shipping_Method so it appears as an option in every shipping zone,
 * the same way Flat Rate and Free Shipping do.
 */

add_filter( 'woocommerce_shipping_methods', 'table_rate_register_shipping_method' );
function table_rate_register_shipping_method( $methods ) {
	$methods['table_rate'] = 'WC_Shipping_TableRate';
	return $methods;
}

add_action( 'woocommerce_shipping_init', 'table_rate_shipping_method_init' );
function table_rate_shipping_method_init() {

	class WC_Shipping_TableRate extends WC_Shipping_Method {

		public function __construct( $instance_id = 0 ) {
			$this->id                 = 'table_rate';
			$this->instance_id        = absint( $instance_id );
			$this->method_title       = 'Table Rate Shipping';
			$this->method_description = 'Charges a rate based on the cart\'s total weight, in tiers.';
			$this->supports = array( 'shipping-zones', 'instance-settings' );
			$this->init();
		}

		public function init() {
			$this->init_form_fields();
			$this->init_settings();
			$this->enabled = $this->get_option( 'enabled', 'yes' );
			$this->title   = $this->get_option( 'title', $this->method_title );
			add_action( 'woocommerce_update_options_shipping_' . $this->id, array( $this, 'process_admin_options' ) );
		}

		public function init_form_fields() {
			$this->instance_form_fields = array(
				'enabled' => array( 'title' => 'Enable/Disable', 'type' => 'checkbox', 'label' => 'Enable this shipping method', 'default' => 'yes' ),
				'title'   => array( 'title' => 'Method title', 'type' => 'text', 'default' => $this->method_title ),
			);
		}

		// Edit the tiers here, or duplicate this whole file with a different $this->id to run a second set of tiers in another zone.
		private function get_tiers() {
			return array(
			array( 'min' => 0, 'max' => 2, 'rate' => 5.99 ),
			array( 'min' => 2, 'max' => 5, 'rate' => 8.99 ),
			array( 'min' => 5, 'max' => 10, 'rate' => 12.99 ),
			array( 'min' => 10, 'max' => 999999, 'rate' => 19.99 ),
			);
		}

		public function calculate_shipping( $package = array() ) {
			$subtotal = 0.0;
			$weight   = 0.0;
			$qty      = 0;
			foreach ( $package['contents'] as $item ) {
				$weight += (float) $item['data']->get_weight() * $item['quantity'];
				$qty += (int) $item['quantity'];
			}
			$value = $weight;
			$cost  = null;
			foreach ( $this->get_tiers() as $tier ) {
				if ( $value >= $tier['min'] && $value <= $tier['max'] ) {
					$cost = $tier['rate'];
					break;
				}
			}
			if ( null === $cost ) {
				return; // No matching tier: the method won't be offered for this cart.
			}
			$rate = array(
				'id'       => $this->get_rate_id(),
				'label'    => $this->title,
				'cost'     => $cost,
				'taxes'    => '',
				'package'  => $package,
			);
			$this->add_rate( $rate );
		}
	}
}