Explosive Powder

The Demolitionist only stocks it after the Wall of Flesh is down, and it exists to make Exploding Bullets. Buy it in bulk on a single visit. The walk to his room is the only real cost, and running dry mid session is annoying in a way the price is not.

They hit hard enough to shove most ground enemies backwards, which is excellent when something is trying to close on you and actively counterproductive when you want a target to hold still. In multiplayer they will knock enemies out of a teammate's line of fire, so think twice before bringing them to a group boss fight.