As it is today scammers have to relay on game mechanics, that not everyone is aware of, wich limit the range and creativity of possible scams. The courier who doesnt know every itsybitsy detail of the latest patch can often fall for it once, regardless how careful he is. So it all ends in a few types of scams that becomes boring quite fast.
My suggestion is to spice it up abit. Couriers should be able to VIEW and VALUE the cargo BEFORE they accept a contract.
This is far more realistic and it would help turn scamming into an art instead of the dull monkey business it is today. It will also punish the hasty and greedy.