Exactly, Matt. I wasn't dealing with the brightest PO worker, but she had stated that the weight to size ratio was off (too light) and if the box was smaller OR THE PACKAGE WEIGHED MORE!! it would have been cheaper.
I'm sure that is not the offical Post Office stance. I have sent passes in larger boxes and not had problems. I believe it has to do with the workers doing their jobs properly. If measured, there is an extra charge. If the size is ignored or not entered in, the normal prices we are used to pop up.
I like Marc's suggestion. The less they do the quicker they will process and may not measure the package. You still may have them measure and charge extra with a printed label. Who knows. I need a cigar.
Many post offices have the scale/label printing thing. I dont know if you can do DC with it though.