Normally the airline approved carriers (atleast the ones that I have seen and the ones that I carry) are 11" tall and under. It doesnt matter if they are so or hard, whatever is comfortable for you and the dog and whatever will go under the seat. 15" does sound high to me and to be honest I probably woudlnt rely on that measurement because like I said, even my largest carriers that I know wouldnt go under a seat are sometimes not 15" high. Sometimes the people at the airlines that you call are mistaken, it has happened to me before where I have been told wrong information so that is smart to want to get another opinion
