The Opera Company of Philadelphia finds its season subscriptions on the decline. So it’s trying something new – four days before a performance, the company releases remaining tickets at half-price. “The availability of half-price seats is made known through periodic e-mail ‘blasts’ much in the fashion of last-minute airline deals. The question remains whether opera fans are the type who will make last-minute plans. Last week’s Il Trovatore opening sold 100 half-price tickets on 24-hour notice.”