Bulk tax is when you have too many of item X in your hold than the station wants to buy. They’ll buy all your cargo but at a lower price point. Doesn’t matter if you want to sell only a certain portion they’ll only buy at the price they set out based on how much is in your hold. I think it’s tied to demand. If it’s high you won’t get the penalty. If it’s medium it’s rougly 50T per unit or half of your maximum hold(I think). If it’s low it’s probably a lot less or just unprofitable. I’ve seen a 50% drop in profit when demand changed from High to Medium even when i’ve had more units.