billing.rb

class Billing < OtherClass

  extend ActiveModel::Naming
  include ActiveModel::Conversion
  include ActiveModel::Validations

  attr_accessor :save_my_billing
  attr_reader :result

  cattr_reader :rate
  @@rate = 0.03

  def initialize(args = {})
    args.each { |key, val| send("@#{key}=", val) }
  end

  def charge_for(item)
    # ...
    log_result
  end

  def reset!
    # ...
  end

  def self.send_billing_report
    # ...
  end

protected

  def charge_custom(item)
    # ...
    log_result
  end

private

  def log_result
    Transaction.create_from_result(result)
  end

end