Cart owner type. Must be either 'User' or 'Guest' if owner_id is provided. Must be empty if shop_session_id is provided.

If owner_type is 'User', this should be the user_id. If owner_type is 'Guest', this can be any unique value, such as a UUID.
and shop_session_id cannot be provided at the same time.

If owner_type is empty, this should be a third party id, such as facebook psid or sc user id. shop_session_idandowner_id` cannot be provided at the same time.