Invoices are sent a month before expiry to give plenty of time to renew your domain name without any interruption to services. Additionally it is a registry requirement that you are provided with a minimum number of expiry notifications prior to the expiry of your domain. Please ensure that your contact email address is up-to-date at all times. You can pay the invoice at any stage and if you do not want to renew your domain you can simply ignore then reminders (in this instance please ensure you have auto-renewals turned off!)