It’s not unusual to find a line of people stretching through the door at Mike’s Pastry in Boston’s North End, waiting for the bakery’s famous cannolis and other Italian treats. Although customers can be reasonably assured of what they’ll get when it’s their turn to order, there’s one thing they won’t find out until it’s time to pay: The price. No matter how hard you look, you won’t find a price anywhere. Not for cookies. Not for cupcakes. Not for the cannoli.