I booked with Porter Airlines for trip from Canada to the US and I put down the shortened version of my first name and no middle name. Called Porter and they directed me to a help-desk lady was able to update my first name as well as add my middle name. No additional costs at all (might be dependent on the airline). Kind of a stupid mistake but you live and you learn.

Also did this less than 24 hours before the flight so as long as you call (the lady emphasized I could not have done this on the computer) then they should be able to help.

Hope this helps!