You should definitely ask when you check in if the flight is full. If it is not full they should allow you to take the seat on the plane; it's much safer for everyone and easier for you. You also may be able to gate check the car seat which will expose it to less wear and tear.
That said, we have checked our travel seat (we don't take the MA traveling, too heavy plus we buy a seat for DS and therefore have to haul it onto the plane every time) when we were on a full flight the one time we didn't buy a seat. I got an Eddie Bauer car seat bag (Babiesrus.com) which seems to work ok; I don't know how hardy it will be over the long term but so far it has held up. According to some of the reviews of the product a Marathon will fit it in so your Wizard should as well.
Marilee
mommy to James
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