I understand the logic behind that thinking, but 
with registered mail and EMS, the packages are 
traceable.  Airmail=no trace, so if someone feels 
your envelope and it feels like money, it's far 
more easier to take off with it. 

In any case, I can only speak from personal 
experience. I feel much more comfortable sending 
cash registered or EMS (I think you get less of a 
chance for crooks with EMS because business 
documents are often sent that way).  I've had a 
lot of Canadian bidders send me cash that has 
never shown up, so I'm not sure I trust plain 
airmail.