Actually I think sending it registered is less safe for the fact you draw extra attention to the envelope.

I have paid for many things on eBay by sending cash in the mail, but of course you need to follow these steps:

1) Place in a sheet of paper and fold it around so the sheet of paper is complete surronding the money.

2) Only send bills, never send coins. (Someone sent me coins in an evelope once and once it got to me, I could easly tell that coins where in there.)

3) Make sure it is addressed correctly and everything like that.

4) Make sure you don't send any more then you are willing to lose. Since sometimes sending it through registered mail with insurance and such could end up costed a lot more then what it is worth. (So for example if the first $20 was lost, then the money you saved by not going registed and such would almost cover the cost of sending again).

Anyway, this is just my opinion - you should do what makes you feel the most secure, but as long as it looks like a normal letter, nothing special about it - it should gain no extra attention and get "lost" on the way.