Optionally allow a shippingMethod to be associated with

a ShippingMethodOption

Resolves #3271

Craft Commerce's ShippingMethodOption erases the information of any
custom ShippingMethod's that were used to create them. While not ideal,
this change sets a new field, shippingMethod on the ShippingMethodOption
class and also proxies the call to getShippingRules to use the attached
ShippingMethod if it is present.

Of course, other functions such as getName, getType, etc will not be
overridden and would need to be accessed like
$option->shippingMethod->getType().

src/elements/Order.php

@@ -1984,6 +1984,7 @@ public function getAvailableShippingMethodOptions(): array
             $option->handle = $method->getHandle();
             $option->matchesOrder = ArrayHelper::isIn($method->getHandle(), $matchingMethodHandles);
             $option->price = $method->getPriceForOrder($this);
+            $option->shippingMethod = $method;
 
             // Add all methods if completed, and only the matching methods when it is not completed.
             if ($this->isCompleted || $option->matchesOrder) {

src/models/ShippingMethodOption.php

@@ -7,6 +7,7 @@
 
 namespace craft\commerce\models;
 
+use craft\commerce\base\ShippingMethod as BaseShippingMethod;
 use craft\commerce\behaviors\CurrencyAttributeBehavior;
 use craft\commerce\elements\Order;
 use craft\commerce\Plugin;
@@ -39,6 +40,20 @@ class ShippingMethodOption extends ShippingMethod
      */
     public bool $matchesOrder;
 
+    /**
+     * @var BaseShippingMethod|null The shipping method this option was derived from.
+     */
+    public ?BaseShippingMethod $shippingMethod = null;
+
+    public function getShippingRules(): array
+    {
+        if ($this->shippingMethod !== null) {
+            return $this->shippingMethod->getShippingRules();
+        }
+
+        return parent::getShippingRules();
+    }
+
     /**
      * @throws InvalidConfigException
      */