Assuming you want the box... I'd be tempted to put the laptop in your hand luggage in an appropriate sleeve, being VERY careful that it isn't under excessive pressure or inadequately padded - i.e. use a proper laptop carrying bag and sleeve and don't over fill or knock it around. Keep the power adapter and other bits in your luggage too, and then ship yourself the empty box in the mail. You can truthfully enough say it's just an empty box worth pennies on the customs declaration on the mail item. As for what you declare in the airport, that's none of my business and I don't even know what you're supposed to do going to Germany.