refill.rb

class Refill
  attr_reader :card, :product, :prices, :details, :total_price

  def initialize(card, product, discount_price = nil)
    @card = card
    @product = product
    @discount_price = discount_price
    @prices = product.prices.map(&:price)
    @total_price = if product.product_type === 'discount'
      set_discount_price
    else
      set_regular_price
    end
  end

  def set_discount_price
    @details = @card.purse_details

    if prices.include?(discount_price.to_i)
      discount_price
    else
      prices[0]
    end

  end

  def set_regular_price
    @details = card.period_details

    card.period_price
  end
end

refills_controller.rb

class Public::RefillsController < ApplicationController
  def new
    card    = Card.find(params[:card_id])
    product = Product.find(params[:product_id])
    @refill = Refill.new(car, product, params[:discount_price])
    
    find_or_create_order
  end
end