To be used as:
cart = TmCheckout::Calculator.new do
# Prices with quantity discounts
fr1 3.11
ap1 5.00, 3 => 4.50
cf1 11.23
# The list of products for buy 1 get 1 free action
___ fr1
end
cart.scan :fr1
cart.scan 'FR1'
cart.scan :AP1
cart.calculate