diff --git a/src/controllers/ShippingCategoriesController.php b/src/controllers/ShippingCategoriesController.php
index 0d27d8d158..00cee0963c 100644
--- a/src/controllers/ShippingCategoriesController.php
+++ b/src/controllers/ShippingCategoriesController.php
@@ -123,11 +123,16 @@ public function actionSave(): ?Response
         $shippingCategory->default = (bool)$this->request->getBodyParam('default');
 
         // Set the new product types
-        $postedProductTypes = $this->request->getBodyParam('productTypes', []) ?: [];
-        $productTypes = [];
-        foreach ($postedProductTypes as $productTypeId) {
-            if ($productTypeId && $productType = Plugin::getInstance()->getProductTypes()->getProductTypeById($productTypeId)) {
-                $productTypes[] = $productType;
+        // If this is the default category, it should be available to all product types
+        if ($shippingCategory->default) {
+            $productTypes = Plugin::getInstance()->getProductTypes()->getAllProductTypes();
+        } else {
+            $postedProductTypes = $this->request->getBodyParam('productTypes', []) ?: [];
+            $productTypes = [];
+            foreach ($postedProductTypes as $productTypeId) {
+                if ($productTypeId && $productType = Plugin::getInstance()->getProductTypes()->getProductTypeById($productTypeId)) {
+                    $productTypes[] = $productType;
+                }
             }
         }
         $shippingCategory->setProductTypes($productTypes);
diff --git a/src/services/ShippingCategories.php b/src/services/ShippingCategories.php
index ff0282aaf4..4bf9481f0f 100644
--- a/src/services/ShippingCategories.php
+++ b/src/services/ShippingCategories.php
@@ -200,6 +200,42 @@ public function saveShippingCategory(ShippingCategory $shippingCategory, bool $r
         // Newly set product types this shipping category is available to
         $newProductTypeIds = ArrayHelper::getColumn($shippingCategory->getProductTypes(), 'id');
 
+        // Find product types that are being removed from this shipping category
+        $removedProductTypeIds = array_diff($currentProductTypeIds, $newProductTypeIds);
+        
+        // Update purchasables to default shipping category when product types are removed
+        if (!empty($removedProductTypeIds)) {
+            $defaultShippingCategory = $this->getDefaultShippingCategory($shippingCategory->storeId);
+            
+            // Get all variant purchasables that currently have this shipping category but whose product type is being removed
+            $purchasableIds = (new Query())
+                ->select(['ps.purchasableId'])
+                ->from(['ps' => Table::PURCHASABLES_STORES])
+                ->innerJoin(['v' => Table::VARIANTS], '[[ps.purchasableId]] = [[v.id]]')
+                ->innerJoin(['p' => Table::PRODUCTS], '[[v.primaryOwnerId]] = [[p.id]]')
+                ->where([
+                    'ps.shippingCategoryId' => $shippingCategory->id,
+                    'ps.storeId' => $shippingCategory->storeId,
+                    'p.typeId' => $removedProductTypeIds,
+                ])
+                ->column();
+            
+            if (!empty($purchasableIds)) {
+                // Update these purchasables to use the default shipping category
+                Craft::$app->getDb()->createCommand()
+                    ->update(
+                        Table::PURCHASABLES_STORES,
+                        ['shippingCategoryId' => $defaultShippingCategory->id],
+                        [
+                            'purchasableId' => $purchasableIds,
+                            'storeId' => $shippingCategory->storeId,
+                            'shippingCategoryId' => $shippingCategory->id,
+                        ]
+                    )
+                    ->execute();
+            }
+        }
+
         foreach ($currentProductTypeIds as $oldProductTypeId) {
             // If we are removing a product type for this shipping category the products of that type should be re-saved
             if (!in_array($oldProductTypeId, $newProductTypeIds, false)) {
diff --git a/src/templates/store-management/shipping/shippingcategories/_fields.twig b/src/templates/store-management/shipping/shippingcategories/_fields.twig
index c0b6c8bce5..be92f16b7b 100644
--- a/src/templates/store-management/shipping/shippingcategories/_fields.twig
+++ b/src/templates/store-management/shipping/shippingcategories/_fields.twig
@@ -43,17 +43,27 @@
     {% set warning = "" %}
 {% endif %}
 
+{% set isDefault = shippingCategory is defined and shippingCategory.default %}
+{% set productTypeValues = isDefault ? productTypesOptions|keys : (shippingCategory is defined ? shippingCategory.productTypeIds : []) %}
+
 {{ forms.checkboxSelectField({
   label: "Available to Product Types"|t('commerce'),
-  instructions: "Which product types should this category be available to?"|t('commerce'),
+  instructions: isDefault ? "The default shipping category is automatically available to all product types."|t('commerce') : "Which product types should this category be available to?"|t('commerce'),
   warning: warning,
   id: 'productTypes',
   name: 'productTypes',
   options: productTypesOptions,
-  values: shippingCategory is defined ? shippingCategory.productTypeIds : [],
+  values: productTypeValues,
   showAllOption: false,
+  disabled: isDefault,
 }) }}
 
+{% if isDefault %}
+  {% for productTypeId in productTypesOptions|keys %}
+    {{ hiddenInput('productTypes[]', productTypeId) }}
+  {% endfor %}
+{% endif %}
+
 {% set defaultInput %}
     {{ forms.lightswitchField({
         instructions: "This category will be used as the default for all purchasables in this store."|t('commerce'),
@@ -80,3 +90,48 @@
         new Craft.HandleGenerator('#{{ nameId }}', '#{{ handleId }}');
     {% endjs %}
 {% endif %}
+
+{% set defaultSwitchId = 'default'|namespaceInputId|e('js') %}
+{% set productTypesId = 'productTypes'|namespaceInputId|e('js') %}
+{% js %}
+    (function() {
+        const defaultSwitch = document.getElementById('{{ defaultSwitchId }}');
+        const productTypesContainer = document.getElementById('{{ productTypesId }}');
+        
+        if (!defaultSwitch || !productTypesContainer) return;
+        
+        function updateProductTypes() {
+            const isDefault = defaultSwitch.classList.contains('on');
+            const checkboxes = productTypesContainer.querySelectorAll('input[type="checkbox"]');
+            
+            checkboxes.forEach(checkbox => {
+                if (isDefault) {
+                    checkbox.checked = true;
+                    checkbox.disabled = true;
+                } else {
+                    checkbox.disabled = false;
+                }
+            });
+            
+            // Update hidden inputs
+            const existingHiddenInputs = productTypesContainer.parentElement.querySelectorAll('input[type="hidden"][name="productTypes[]"]');
+            existingHiddenInputs.forEach(input => input.remove());
+            
+            if (isDefault) {
+                checkboxes.forEach(checkbox => {
+                    const hiddenInput = document.createElement('input');
+                    hiddenInput.type = 'hidden';
+                    hiddenInput.name = 'productTypes[]';
+                    hiddenInput.value = checkbox.value;
+                    productTypesContainer.parentElement.appendChild(hiddenInput);
+                });
+            }
+        }
+        
+        // Listen for lightswitch changes
+        defaultSwitch.addEventListener('change', updateProductTypes);
+        
+        // Also listen for the custom lightswitch event
+        $(defaultSwitch).on('change', updateProductTypes);
+    })();
+{% endjs %}