Express Mail Service (EMS) was first available in Cambodia in 1995, it is faster than express service (red label) which has always been offered by the post office.
On 20th Feburary 1997 Cambodia Post issued a set of 6 songbird stamps, all of them carry the EMS Khmer Express logo in the background:
However the issue is not highlighted by advert as special EMS stamps, philatelic pamphlets just describe it as songbird thematic:
Front and back of the philatelic pamphlet.
As a matter of fact the denominations are awkward, they are too low for EMS yet too high for usual tariff, consequently it makes the set not competent for use and quickly end up at the philatelic counter as souvenir only.
The philatelic counter is more like a tourist souvenir counter, stamps are sold at multiples of face value for customers to collect or use postally. With no surprise the "souvenir" price of this EMS set is high enough which discourages people from consuming these birdies on mail, and therefore postal used copies are very rare.
EMS Khmer Express has never used postage stamps for payment. Below is a typical EMS bag for letters and documents: