I disagree mate )). In this context we use "get" in the sense of receiving or obtaining something; a very common use of the word in question. As the informative video that Bakero linked makes clear, no cafe worker is going to think that the customer who asks this wants to go behind the counter to make their own drink: In the situational context, that would be absurd. They would know, being flexible users of the language, that the customer wants to obtain a coffee from the worker.
All about the illocutionary force of an utterance, innit.