Buy X quantity of a product, Get $Y off

gistfile1.txt

DISCOUNTS_BY_QUANTITY = {
  10_000 => 20,
  1_000 => 15,
  100 => 10,
  10 => 5,
}

Input.cart.line_items.each do |line_item|
  next if line_item.variant.product.gift_card?

  quantity, discount = DISCOUNTS_BY_QUANTITY.find do |quantity, _|
    line_item.quantity >= quantity
  end
  next unless discount

  message = "$#{discount} off when buying at least #{quantity}."
  line_item.change_line_price(
    line_item.line_price - (Money.new(cents:100) * discount),
    message: message,
  )
end

Output.cart = Input.cart