I had the same thing happen when I won an Energon multi-pack (yellow/red Optimus and Overload) - the person got a quote from the (post office?) in its mint-boxed state, and when they took the toys out of the box and folded it up, they got a much cheaper quote from the (post office?) because it was now in a smaller mailing box.
Maybe it was UPS, and not the post office, because I noticed that UPS ask for dimensions when calculating shipping, while USPS don't.

Besides, if the lego box is as big as I think it is, the USPS probably wouldn't accept it, because it's dimensions would be larger than their current limits (a lot smaller than they used to be - as I had trouble posting home boxes from BotCon that were the same size as the previous year). As such, UPS would probably be the only option to ship it.