order.rb

class Order < ActiveRecord::Base
  has_many :line_items

  validates :total_price, numericality: true

  before_create :apply_returning_customer_discount, if: lambda { |order| order.returning_customer? }

  def apply_returning_customer_discount
    self.total_price = self.total_price * 0.9
  end
end

Oh my... not only is the grammar horrible, but I mixed several different drafts, and some crass language. Please, allow me to try again:

Why use callbacks at all? Rails' notion of callbacks are a hack which allow you to be lazy and not think about the domain; instead delve more deeply into the problem space and work to uncover a better abstraction. In domain language - what the heck is an Order? To me, an Order is probably an artifact that only exists after one has finished a purchase. After you've "checked out."

Maybe a better abstraction is a Cart which contains a collection of Items, or SKUs a customer intends to purchase? A Cart could delegate to a PriceRegister or some form of calculator to sum totals, and then after checkout an Order, or perhaps Receipt, or maybe BillOfSale could be stored for later use.

s/crass/snarky/