Have you ever come across such cover? It is call the first flight cover. A "First Flight Cover" is a commemorative envelope for an airlines inauguration of a new route or service, which is similar to the FDC which commemorative for issue of new stamps. They can be anywhere from plain envolope to very nicely designed covers with colorful (sometimes pertinent) stamps and cancellations.
First flight cover normally come in pairs. For examples, please see the 2 cover below:
Both the covers were to commemorative first flight of the Malaysia-Singapore Airline to Perth and vice versa. Cover A was sent from Singapore to Perth while Cover B was sent from Perth to Singapore. Both the cover affixed with the actual postage cost required for such destination at that time. Thus, they are valuable items for aerophilately research. From my experience, matching pair of FFCs are really hard to come by.
I have not come across any documentation/catalogue which outlines all the FFC in Malaysia yet. I hope those who are specialized in this field could one day publish more information for Malaysia FFC such that it benefits all stamp enthusiasts.