Billy Wagner departed Shea Stadium at about 3:30 yesterday, and he did so with the Mets' first formal offer in hand. According to an official from a National League club, the offer was a three-year, $30-million package. Given how badly the Philadelphia Phillies want to retain their lefthanded closer, the Mets could very well need to raise their bid in order to bring Wagner to New York. The Mets didn't want to commit three years to Wagner, not when all the Phillies have floated is a two-year commitment with an option for a third. And the $10-million annual average value falls short of Mariano Rivera's industry-setting $10.5 million, as per the two-year, $21-million extension he has through 2006. As even Wagner himself said yesterday at Shea, "Mariano's the best."