Allows [merging](/../api/customers-overview#cart-merge-during-sign-in-and-sign-up) items from an anonymous Cart into the most recently modified active Cart of a Customer.
If no active Cart exists, the anonymous Cart becomes the Customer's active Cart.
If the Customer has multiple active Carts, the anonymous Cart is merged into the most recently modified active Cart.
If the Customer exists in the Project but the `stores` field references a different [Store](ctp:api:type:Store), this method returns an [InvalidCredentials](ctp:api:type:InvalidCredentialsError) error.
Authenticates a Customer associated with a Store.