Q: why does the buckler exist?
Serious question! It's worse than a shield but uses the same proficiency. The price difference is inconsequential.
In the old version of shield master, it at least could be used as it was in history: as a light weapon in conjunction with a sword or other weapon. The Sept playtest got rid of that.
So: why does it exist at all?
Recommend it be added to the martial weapon list as a d4 weapon with the Light quality. Or at least, restore that benefit from Shield Master.
Otherwise, if it is merely a worse shield, then recommend you simply remove it from the game as it serves no purpose. And let players describe their shields as bucklers if they wish.