I think the implication is that the customer is drinking alcohol frequently lately because the lady ordered in the way you would order many alcoholic beverages with ice.
"and so" is perfectly valid as a conjunction for implying causation. "Thus" would be a synonym. It fits better than "and also" which doesn't imply causation and so isn't the right word.
It's not that they didn't know Starbucks secret code ("iced" is a common term to use for putting ice in any drink). It's that they used alcohol code instead ("on the rocks" is a common term to use for putting ice in alcohol).
"Secret code" or "common term" so what? I've never said the word "iced" in my life. Guess that means someone will post some nonsense about me because of my vocabulary.