script.rb

BLOCK_ID = "631f8edae5f9ce785673920c" # Replace with block id
DISCOUNT = 0.90
DISCOUNT_MESSAGE = "Checkout offer"

Input.cart.line_items.each do |line_item|
  next unless line_item.properties.has_key?("_checkoutblocks")
  next unless line_item.properties.has_value?(BLOCK_ID)
  line_item.change_line_price(line_item.line_price * DISCOUNT, message: DISCOUNT_MESSAGE)
end

Output.cart = Input.cart