I am going to be flying Airtran with my 20 lb Norfolk ter. mix. I bought the Med. size Sherpa bag with the Delta logo on it. I am wondering if anyone has recently flown with and in-cabin pet and what your experience was? I am worried because i could not find a bag that met the size requirements. I did find that the Sherpa "Guaranteed on Board" accepted my bag size. So, I have a printed out "Guaranteed on Board" certificate. Does this mean anything to the ticket agent especially if the bag is not the size on the Airtran website?