The Unlimited International texting:
Includes messages sent from Canada on the Fido network to email addresses or Canadian, U.S. and international wireless numbers. Sent/received premium messages (alerts, messages related to content and promotions) and messages sent while roaming not included and charged at applicable rates. ~ taken from Full Details.
Generally, there shouldn't be any additional charges for sending/receiving SMS to/from short code numbers. However, there are some premium services which would incur extra charges. Those charges would be for the services and not necessarily for sending/receiving the SMS per se (see here).
Hope this helps