Train your clients that April 15 (April 18, this year) is just another spring day.  The world does not come to an end if their return is not filed by May 12 or June 23.  Tell them you don't charge more for your help getting them a 3% government loan.  If they don't think enough of you to accept an extension, let them go elsewhere that caters to brainwashed adherents to artificial deadlines.