Let's assume that you have a domain domain-for-ssl.com and a shop at shop.domain-for-ssl.com
You can protect both of these pages with one SSL certificate, but it must be a WILDCARD certificate. We offer just such a certificate - Rapid SSL Wildcard.
However, you should know that the certificate does protect subdomains, but only the first-level. This means, that it will work at the address https://domain-for-ssl.com and https://shop.domain-for-ssl.com, but it will no longer protect the address https: //www.shop.domain- for-ssl.com, because such an address is already a second-level subdomain (to put it as colloquially as possible - it has two dots before the address of the domain itself).