From: Rabbi Moshe Sofer
To: Rabbi Shimon Schattin
The Chasam Sofer rejects the theory that the father acts as a forced agent (shaliach ba'al korcho) for his minor son in pidyon haben, arguing the primary obligation rests squarely on the father from the verse 'kol bechor banecha tifdeh.' He explains the custom to perform pidyon personally as rooted in the principle of 'ani v'lo ha-shaliach,' commemorating God's direct involvement in the plague of the firstborn, though this does not constitute an outright prohibition on using an agent.
Source: Shu"t Chasam Sofer YD §293