It is a separate entity from China Post. For inland Hong Kong, the price is 1.40$ and for international postage by air mail it is 2.40$ for Zone 1 and 3$ for Zone 2 where here is the list for the zoning but Europe and the USA is on zone 2 and Singapore and Taiwan are in zone 1 for instance.
Hong Kong Post is very efficient but it is expected for such a small place!
Of course you can find in Hong Kong the usual courier companies like: