Have you ever heard of a money order? If you must send things through the mail at least secure it by making it a money order.
Of course I have heard of money orders, and if you look at my trading thread you'll see I've offered this. I have had only one person take me up on the offer of a money order, and they guy apparently did not cash it himself, but used it to pay a bill.
Have you ever tried going to the post office and asking them to cash a $1000 money order? They will only do it if they have enough cash on hand, which the smaller post offices usually don't. Someone who pays $20 for overnight mail probably doesn't want to wait another day or more to cash the money order.
It's really friggin stupid to send cash itself because it could be stolen or lost at any point and you won't get reimbursed. Even if you buy insurance the burden of proof is on you, and that's real hard to prove.
I am not stupid. I have been doing this for a long time and I can work out the economics. The loss rate for sending things in the mail (not just cash, but all sorts of things, eg online auctions) where someone wasn't obviously trying to scam me, is around 2% or 1 in 50. Occasional losses are a cost of doing business, and I'm not going to spend a huge amount of time chasing a small amount of money.
In the one and only case I have dealt with where someone claimed that a money order was lost in the mail (not bitcoin related) the recipient later found the lost money order and tried to cash it, which led to a big mess. I'd really rather not go through that again.