We are paying an ever increasing cost but it seems Sonic is reducing the number of features a subscriber gets.
Typically you would get 15 (main account and 14 mailboxes) if you have a connectivity service like Fusion DSL or Fiber. If you go over the standard mailboxes it is $6 for 4 additional mailboxes.
So I am not sure how you had 18 mailboxes before. Did you have a hosting service? Hosting gives you access to more mailboxes.