> ## Documentation Index
> Fetch the complete documentation index at: https://docs.supercycle.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Checkout rules

> Turn on Supercycle's checkout validation to prevent overbooking and other rental checkout errors

Checkout rules are Supercycle's checkout validation in Shopify. The validation runs against every cart and blocks checkout when a rental, membership, or availability requirement isn't met, so a customer can't book an item that isn't available or remove a required deposit. Keep it turned on.

The rule is turned on under **Checkout rules** on <Icon icon="shopify" iconType="solid" /> **[Checkout](https://admin.shopify.com/settings/checkout)** in your Shopify settings.

***

## Validations

The Supercycle rule checks the following before a customer can complete checkout:

* **Unavailable items.** Requested quantities don't exceed available inventory for rental items.
* **Deposit requirements.** Required [deposit](/documentation/configuration/risk/deposits) products are in the cart for rental items.
* **Card authorization.** A vaulted payment method is available when the rental requires one.
* **Account requirement.** The customer has an account for membership rentals, unless they're buying a membership plan.
* **Multiple memberships.** Only one membership line in a cart.
* **Membership combinations.** Membership rentals and non-membership rentals aren't mixed for the same item.
* **Credit allowance.** The customer doesn't exceed their membership credit limit.
* **Start date consistency.** Items with the same selling plan have matching start dates for a given duration.
* **Fixed dates.** When fixed dates are on, every rental item in the cart has matching start and end dates.
* **Fulfillment constraint.** Rental items are fulfilled from the location the customer selected at checkout.

***

## Turn on checkout rules

<Steps>
  <Step title="Open checkout settings">
    Go to <Icon icon="shopify" iconType="solid" /> **[Checkout](https://admin.shopify.com/settings/checkout)** and find **Checkout rules**.
  </Step>

  <Step title="Turn on the Supercycle rule">
    Turn on the Supercycle **validation** rule.
  </Step>

  <Step title="Save">
    Select **Save**.
  </Step>
</Steps>