I am very confused as to whether instant messaging (AIM) is included in my Blackberry Unlimited Plan (19.99).

In the store the salesperson told me it is unlimited because it is covered under web browsing. But the CS agent on the the phone says that it is not included because it is sent as a text message. Who is right?? I dont want to rack up a $400 bill on my first month, but I really would like to use AIM without paying per message!

Help!