Hmmm... page 129, under "Convenience Foods" says :

"Although it is important that you eat primarily unprocessed foods, some controlled-carb food products can come in handy when you are unable to find appropriate food, can't take time for a meal, or need a quick snack."

He goes on to caution you to read labels carefully and not overdo it, but in my interpretation, that would allow for the occasional shake if the carb count is low enough.