order.rb

class Order::PurchaseService
  attr_accessor :cart, :user, :credit_card

  def initialize(cart, user, credit_card)
    @cart = cart
    @user = user
    @credit_card = credit_card
  end

  def perform
    transform_cart_into_order(@cart)
    calculate_tax!
    calculate_shipping!
    charge_credit_card
    send_order_request_to_warehouse!
    email_invoice_to_user(current_user)!
  end

  ...
end