The weird obsession with "correct posture" causes way more harm than good. What quaks espouse as the "correct posture" is a generalisation, and most likely not at all the correct posture for you as an individual. On top of that, obsessing over your posture most likely causes you to tense up, and tensing up will make any posture damaging.
Seriously. Go to a fucking trained specialist before spending ungodly amounts of money on overpriced chairs that don't help you. It's a scam.