Sometimes it happens that the person who made the booking claims that he did not receive the confirmation ( usually the email is in his SPAM mailbox ), or deleted it by accident. In such a situation you can send the ticket again from the system, or download a .pdf file and send it manually. All you need to do is to search for the correct email in the system.
If the email cannot be located, it means that the transaction has not been completed correctly. Each transaction is also saved in the Stripe system.
