cos the QM will open the box - take what he wants and give you what he cannot swop/sell/barter in the Sgt's mess
RL showing his prejudice against and ignorance of the army here, rather than any 'top tip' for, as any fule kno, the QM resides in the officers' mess.
I'd pay more attention on my next exchange tour RL, if I were you. Who was it with anyway, the AGC?