Added get_rate_id() method for consistent rate nams

includes/abstracts/abstract-wc-shipping-method.php

@@ -207,13 +207,34 @@ abstract class WC_Shipping_Method extends WC_Settings_API {
 		return $this->rates;
 	}
 
+	/**
+	 * Returns a rate ID based on this methods ID and instance, with an optional
+	 * suffix if distinguishing between multiple rates.
+	 * @since 2.6.0
+	 * @param string $suffix
+	 * @return string
+	 */
+	public function get_rate_id( $suffix = '' ) {
+		$rate_id = array( $this->id );
+
+		if ( $this->instance_id ) {
+			$rate_id[] = $this->instance_id;
+		}
+
+		if ( $suffix ) {
+			$rate_id[] = $suffix;
+		}
+
+		return implode( ':', $rate_id );
+	}
+
 	/**
 	 * Add a shipping rate. If taxes are not set they will be calculated based on cost.
 	 * @param array $args (default: array())
 	 */
 	public function add_rate( $args = array() ) {
 		$args = wp_parse_args( $args, array(
-			'id'        => '', // ID for the rate
+			'id'        => $this->get_rate_id(), // ID for the rate. If not passed, this id:instance default will be used.
 			'label'     => '', // Label for the rate
 			'cost'      => '0', // Amount or array of costs (per item shipping)
 			'taxes'     => '', // Pass taxes, or leave empty to have it calculated for you, or 'false' to disable calculations

includes/shipping/flat-rate/class-wc-shipping-flat-rate.php

@@ -123,7 +123,7 @@ class WC_Shipping_Flat_Rate extends WC_Shipping_Method {
 	 */
 	public function calculate_shipping( $package = array() ) {
 		$rate = array(
-			'id'      => $this->id . $this->instance_id,
+			'id'      => $this->get_rate_id(),
 			'label'   => $this->title,
 			'cost'    => 0,
 			'package' => $package,

includes/shipping/free-shipping/class-wc-shipping-free-shipping.php

@@ -146,7 +146,6 @@ class WC_Shipping_Free_Shipping extends WC_Shipping_Method {
 	 */
 	public function calculate_shipping( $package = array() ) {
 		$this->add_rate( array(
-			'id' 	  => $this->id . $this->instance_id,
 			'label'   => $this->title,
 			'cost' 	  => 0,
 			'taxes'   => false,

includes/shipping/local-pickup/class-wc-shipping-local-pickup.php

@@ -56,7 +56,6 @@ class WC_Shipping_Local_Pickup extends WC_Shipping_Method {
 	 */
 	public function calculate_shipping( $package = array() ) {
 		$this->add_rate( array(
-			'id' 		 => $this->id . $this->instance_id,
 			'label' 	 => $this->title,
 			'package'    => $package,
 		) );