Spree Commerce: Skip delivery checkout step

checkout_controller_decorator.rb

Spree::CheckoutController.class_eval do
  def before_delivery
    @order.next
    redirect_to checkout_state_path(@order.state)
  end
end

order_decorator.rb

Spree::Order.class_eval do
  state_machine do
    after_transition :to => :delivery do |order|
      if order.shipments.all? { |shipment| shipment.shipping_rates.count == 1 }
        order.shipments.each do |shipment|
          shipment.selected_shipping_rate_id = shipment.shipping_rates.first.id
        end
        order.next
      end
    end
  end
end