In Cambodia no postage due stamps have been issued since postal service resumed in 1979, in fact it is unnecessary to have them because almost all mail senders take their mail to post office to pay and send, occurrence of postage underpayment by senders (I am not making myself stupid by stating "by senders", it can be done by POSTAL CLERKS) is rare.
However a "T.P" (acronym for French "taxe perçue") handstamp cachet is in use, it does not serve the purpose of postage due or postal tax, it tells postage is collected.
This 1991 cover gives a good illustration on the usage. During the inflation period of late 1980s and early 1990s, some post offices were short in postage stamps which caught up with inflation. This cover postmarked 30th November 1991 was franked with one obsolete 3r stamp only, the actual postage of 200r was paid right at the post office counter. "T.P" indicates postage has been fully paid at counter.
Below is a recent sample. On this 2009 cover only one 1500r stamp was affixed, the left out 800r postage was paid at the counter. "T.P" was used to mark full postage paid.
Sometimes "T.P" is used on official mail. Below is from the Ministry of Posts & Telecommunications, "T.P" once again served as postage paid indicator.