rottler:I would check with a local store that carries them and ask them if they would be willing to exchange it for the xs for you, the worst they can say is no and if they let you then you wont have to deal with the shipping and all.
This. We got a car seat and extra base from Amazon and Overstock for Christmas, and then exchanged both without receipts or anything to Buy Buy Baby. The packages even had stickers on them that said where they came from if the clerk looked, so I'm assuming they didn't look or just didn't care :) Plus if you bring it to a store, you can try on other sizes to be sure.