How to send a letter without parents knowing?

There is a YouTube channel I like so I want to send them a fan letter because I love their videos. I just want to send them a simple envelope with a simple letter inside without parents knowing because they're overprotective, I'm NOT telling them about this. I think I can get my hands on some envelopes and MAYBE stamps. But where do they sell stamps apart from post office, and what stamps can you use for overseas mail? (I'm in Australia, the Youtubers are in America). One of those letter drop box bins is literally a 2 minute bike ride away from my house so delivering the letter will be easy. What I want to know is what stamps can I use and where to get them.