Hi,
It really depends on the weight of your letter/package. Its best that you go to the Post Office and have them weigh it and buy the postage on the spot.
If its just a letter, it not expensive, a dollar plus only...
Btw, please note that you would not get a reply from Adam unless you provide a self-addressing envelope with US stamps of it (which is quite impossible to get in Singapore). So please be prepared NOT to get a reply from Adam.
Another point - Adam would not be able to read your letter till a few months later - he is on Tour and I think he is going through his mail only if he happens to be in LA. (Although his Mom might be going thru them for him.)