Who knows what a foul on a post player is in the NBA? If somebody tells you they do, tell 'em to watch Shaq run people over for a while.
High school-- I'm not sure if the CIF uses NFHS rules or modified NCAA rules, but it doesn't really matter. It's always your foul if you lower your shoulder and push off your opponent. It also doesn't matter if your defender is moving or has his feet set either. The general rule is that if you lower your shoulder and contact the defender in the torso, it's your foul. Iow, good luck to you if you try that move.
Forget the "no charge" circle. That only applies if the CIF is using womens NCAA rules. Even then, it doesn't sound like that rule is applicable to what you're describing anyway.
Welcome to the forum.