For that, you need to buy a wildcard certificate for the first level subdomain. So, in the above example, to secure refunds.billing.mysite.com (second-level subdomain), you need to buy a separate wildcard certificate for *.billing.mysite.com (first level subdomain). In other words, you must list the first level subdomain as your primary domain.     Cost-saving tip: If you have only a handful of second-level subdomains, you can get a multi-domain SSL certificate. If you have only one or two second or even third level subdomains, simply buy separate single-domain SSL certificates for them to save the money!