gistfile1.rb

# Shipment
def after_ship
  Spree::Config.shipment_handler_class.factory(self).perform
end

module Spree
  class ShipmentHandler
    class << self
      def factory(shipment)
        if sm_handler = "Spree::ShipmentHandler::#{shipment.shipping_method.to_s.split('::').last}".constantize rescue false
          sm_handler.new(shipment)
        else
          new(shipment)
        end        
      end
    end
    
    def initialize(shipment)
      @shipment = shipment
    end
    
    def perform
      @shipment.inventory_units.each &:ship!
      @shipment.send_shipped_email
      @shipment.touch :shipped_at
      @shipment.update_order_shipment_state      
    end
  end
end

module Spree
  module ShipmentHandler
    class DigitalDownload < Spree::ShipmentHandler
      def perform
        # Custom logic here
        super
      end
    end
  end
end

# Or totally override everything and set a new Shipment Handler and write a custom factory method
module App
  class ShipmentHandler
    class << self
      def factory(shipment)
        # Custom logic
        # The only requirement forced by the implementation is that object returned responds to a perform method
      end
    end
  end
end