If you have received an invoice for a domain that you no longer require you should login to my.xilo.net, go to your domain list, and disable auto-renewal. This will remove any due invoice and will not renew the domain.
Once auto-renew is disabled, the domain will expire and will be available to be registered by anyone. If you then decide you want the domain, you will need to attempt to re-register it. If at this time you find out that another party has purchased the domain, there is no way for you to regain ownership of the name.
If you require further assistance please contact our support team.