My guess is that they have the ability to allow clients to host more than 1 domain, it's just they don't let you... unless of course someone is willing to pay extra to host additional domains.
Therefore, it sounds to me like the email they sent to you about an additional domain is just trying to up-sell.
In other words, let's hypothetically say you're paying $5 for hosting 1 domain. Now instead of charging you $10 for hosting 2 domains, the "reduced price" would be something like $7 (for both domains).
Of course, I am guessing about all of this and I could be way off
