Yes, I was just going to say... you can check 24 bags for free! Even with car seats thrown in there, you'll have plenty of luggage space. I would go to Goodwill/yardsales and find a cheap suitcase that the luggage carts will fit in. Stick the carts in the empty suitcase at the Southwest desk, then give duffles to SW, then cart-suitcase and car seats. I also like the suggestion of a hand-held luggage scale. Yes, you can try to weigh the suitcases yourself, but it's kind-of a pain. I've seen the scales at Target for about $10 and am going to get one myself. We fly several times a year, and it's annoying for DH to have to weigh himself, then get on the scale with the suitcase so I can read the combined weight, then subtract to see how heavy the packed suitcase is. I always like to be a couple of pounds "under" to allow some wiggle room. You also might want to take an empty duffle in the cart-suitcase (if it won't make the cart-suitcase overweight, and if using it won't put you over your 24 checked-item limit); that way you'll have space for souvenirs and room to shift things around, just in case anything IS overweight.