Single SSL Certificate, does cover my web address with "www" and without?

"If I order a single SSL certificate for my storefront, will it cover my store's web address both with and without "www"?

Yes, it will.

Let's say your domain is "Example.com". A single SSL Certificate will cover both "Example.com" and "www.Example.com". When Aleyant set up the SSL Certificate, we would usually redirect "Example.com" to "www.Example.com" when configuring the SSL Certificate.

A SSL Certificate is required when the store is collecting credit card information via Payflo Pro or Authorize.net
 
As of 2017, secure certificates can be enabled on any Pressero site and installed automatically for any site. Any old certificate ordering information no longer applies. Please check announcement and instructions on articles below:
Change to the Automatic Certificate System (HTTPS/SSL/TLS) for Pressero
What do I do when I'm ready to "go live" with my Pressero site?