# How do I set a custom item quantity totalizer for shipping rates?
Shipping rates can define a custom item quantity totalizer to control how item quantities are calculated when determining whether a rate is applicable.
For example, you may want sample products to be excluded from the item quantity used when evaluating quantity-based shipping rules.
First, define a setting on the variant:
```php
\Aero\Catalog\Models\Variant::settings(function (\Aero\Common\Settings\ModelSettingGroup $group) {
$group->boolean('is_sample')->default(false);
});
```
The `setItemQuantityTotalizer` method can then exclude sample products when calculating the total quantity:
```php
\Aero\Cart\Models\ShippingRate::setItemQuantityTotalizer(function ($items) {
return $items
->reject(function (\Aero\Cart\Models\OrderItem $item) {
return $item->buyable->setting('is_sample');
})
->sum('quantity');
});
```
The totalizer receives the order items and the shipping rate as parameters and should return the total quantity of the items or `null`.
Returning `null` means that the default item quantity totalizer will be used.
---
### Parameters
| **Parameter** | **Type** | **Description** |
| `$items`
| `\Aero\Common\Eloquent\JavaScriptableCollection`
| The order items being evaluated. |
| `$rate`
| `\Aero\Cart\Models\ShippingRate`
| The shipping rate being evaluated. |