I have send an envelope with a card, a t-shirt and selfmade key chains.

The receiver got the envelope, -without- the key chains.

What does this mean? Did they steal the key chains (worth like 30 baht) or is it just not allowed to send stuff in an envelop?

I'm a bit worried, as I recently sent an envelope with a diary and a memory stick...

From where (which CM branch) to what city or country ?

CMU post office to Austria.

Speaking from my vast (not) personal experience as a temporary worker sorting mail within the U.S. postal system, I can tell you that often people will stick items into an envelope without providing any padding or restraints on movement of the item within the envelope. The item shifts during handling and damages the envelope. Also, if it's not readily apparent that the envelope contains something besides documents or magazines, it can be sent thru the automated sorting equipment which will do a real number on an envelope with a small item inside. Don't know if they use automated handling equipment in Thailand.

The problem could have occurred on the receiving end, too. Perhaps the envelope was damaged by handling equipment in the receiving country or perhaps the customs people wanted to inspect the envelope to see if duty should be paid.

Did it look like the envelope was damaged in transit? Was there a sticker from customs indicating that the envelope had been opened for inspection?

In the future, wrap the items in bubble wrap and use a padded envelope.