My experiences with them in the past is that they packaged these things up, wrote the box number on them and put them in the stockroom.
I also think this is done by a handful (if not, less) of people and I am more than certain they get these questions constantly. I can understand their end, not wanting to open up the box just to see if you can fit more in them. Constantly trying to play shipping tetras on boxed up stuff can get time consuming really quick.
The boosted shipping cost may be a leftover habit from ebay days gone by where they hide some of the product cost in the shipping charges.