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 an account with the given credentials is not found, an [InvalidCredentials](ctp:api:type:InvalidCredentialsError) error is returned.
Authenticates a global Customer.