"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