I would also recommend looking into your domain registrar and find out if they have the option to consolidate your domains. By doing this, you will sync all of the domains you select to renew on the same date. That way you do not have to keep track of which is expiring at which date and making sure you renew and invoice on time.
Personally I always offer my clients the option to pay for their hosting and domains on a yearly basis (your domain is good for a year, why not renew your hosting for the same period). So by doing this, I can bill them for everything once, all of their domains renew the same day and only need to remember one invoice per year.