20140126034036_create_purchases.rb

class CreatePurchases < ActiveRecord::Migration
  def change
    create_table :purchases do |t|
      t.string :name
      t.integer :shipping_fee

      t.timestamps
    end
  end
end

purchase.rb

class Purchase < ActiveRecord::Base
  validates :shipping_fee, inclusion: { in: ShippingCharges.all }
end

purchase_spec.rb

require 'spec_helper'

describe Purchase do
  it { should ensure_inclusion_of(:shipping_fee).in_array(ShippingCharges.all) }
end

shipping_charges.rb

class ShippingCharges
  def self.all
    [0,8,14]
  end
end