Mmars
Well said. I agreed that cash is not insurable but if you are posting it out as documents, I do not see that as an issue. Of course, nothing is 100% safe, even if you have the notes personally delivered by a friend or mate. Believe me, I lost some notes like that. Never called them friends, unless you really know them well.
In a registered post, even it is registered, there is still some risk involve. You still need the two posties to do their job right!
Whilst I note that you may not be able to claim compensation for cash lost in the mail, have you tried claiming them as numismatics or collectable? Customs may be also an issue there, and given that boarder securities is a major concern for every Western countries, what they are interested in are illegal drugs or prohibited substances. Australian Customs are also looking for import duties too. At least this is what the Australian Customs are looking for. Few years ago, I had a mail opened for inspection by the Australian Customs. The contents were well over 40+ notes. It was from Italy, but all the contents were intact. Nothing missing (touch wood). The sniffing cute little dog must be on annual leave that day!
Whichever way you want to send, sometime, an ordinary airmail is still the best, but do you want to risk that either?
From my experience, certain countries are really excellent with postal delivery. Australia is one of them. All you need is the house no, street name and the post code. China is another one. Unfortunately certain countries are famous in postal thieves. Even in US, which I lost one recently. Mail was cut opened what looks like opened by a box cutter. The contents went missing and USPS then sent me the empty envelop using their USPS large envelop. This was posted via normal air mail.