I would guess that a large number of those kits are smaller rubber powered models so the boxes will be much smaller than your estimate. If he has some large scale kits as well then I would use a number about 60 to 75% of the volume you used in your colculations.

Somebody did this to me once and prevented me from getting an award I was well deserving of. The award stated that I spent over 1,500 hours of my personal time developing software. Somebody did the math and said there was no way I could have done that.

The fact is I spent over 3,000 hours working on and told the person asking that I did less because I knew nobody would believe that number. It was over an 8 year time span - not the 3 years the person doing the math had suggested. Unfortunately, I didn't learn about all this until it was too late to provide any insight into what I had done.