Shipping is cheap enough to actually ship rubber ducks short distances, which means that the vast majority of cargo will be equally prosaic.
Shipping 25 air/rafts will not net you anything like MCr 15 unless you stole them, and then you have to fence them for less than nominal value.
But if your point is that the ship is much cheaper than the trade volume it carries, then I of course agree. It obviously has to be to make shipping affordable. Equally obviously the ship is generally more valuable than the average cargo.