10 years back a number of registered envelopes were sent to me from the UK none of which arrived, on asking the UK post office what happened was informed that they were stolen in Thailand.

If the UK post office knows of this problem why don't they tell you before accepting the registered envelope ?

Bulky envelopes or packages routinely disappear.

Two times I've sent cash to Thailand from Japan. Both times in a plain brown envelope and the money was wrapped in plain white paper so the cash could not be seen from the outside. Both times, it never arrived at it's destination.