After investigating this a lot more I finally found a gem that works with the Mac Appstore receipts: itunes-receipt
require 'itunes/receipt'
# pass Base64 encoded raw receipt data which you received from your app
receipt = Itunes::Receipt.verify! 'ewoJInNpZ25hdHVyZSIgPSAi...'
receipt.product_id # => 'com.example.products.100gems'
receipt.transaction_id # => '1234567890'