Another thing that you may ask the vet about is Children's Benadryl for their upper respiritory infection, based on their weight it can ease their sneezing and coughing.....
On the guarantee from Petland....not sure, but doesn't Florida have pet lemon laws. You might want to look into that.
Just checked it out....Florida has one of the toughest pet lemon laws in the country
http://www.consumeraffairs.com/pets/lemon_fl.html