Open order by unregistered - how to make payment?


Some my customers do not finish with payments on some reasons and orders are Open. If a customer is a registered user this is OK - I send them a notification and ask to login and to try to make payment again. But how to be with unregistered users? As they do not have an account they can not make any manipulations with their Open orders. How do you solve this situation with Open orders? Thanks. Michael