Buy X and Y, Get 100% off Z

discount_product_ids = [10593412163,301378967] # Enter qualifying product_id
free_product_id = 359354662944

valid_product = false
Input.cart.line_items.each do |line_item|
  product = line_item.variant.product
  valid_product = true if (discount_product_ids.include?(product.id) or valid_product)
end

if valid_product
  Input.cart.line_items.each do |line_item|
    next unless line_item.variant.product.id == free_product_id
    line_item.change_line_price(line_item.line_price * 0.0, message: "Free item with purchase")
  end
end

Output.cart = Input.cart