What is the difference between postural hypotension and orthostatic hypotension?

Semantics: They mean the same thing. Postural hypotension theoretically could mean any change in posture that induces low blood pressure, but in reality this is only seen when assuming a standing-up position from seated or lying down. Orthostatic hypotension describes the same phenomenon.

Same thing: Two ways of expressing the same issue, a fall in BP when standing.

Almost same: Positional hypotension refers to decrease in blood pressure with standing. While orthostatic hypotension refers to a defined decrease of BP drop and increase in heart rate with standing from a recumbent position.