[nagdu] Airline Baggage Question

[sarah baebler]

Hi all,
I have a question.  I love Northwest and have always flown with them in the past and they never charged for bags until Delta merged with them.  Now when I'm heading to Washington Seminar this will be the first year I will have to pay to check my luggage.  I have a big dog who eats 6 cups a day and we are staying for a week so that is A LOT OF DOG FOOD.  I was talking to my friend and she also is going and has said that if you are bringing a service animal and have items in your bag for the animal they will not charge you for your bag.  Has anyone had experience with that?  Or know if you have to pre-arrange that?  I don't want to show up and have a debate with the clerk.  
Sarah and Dante