While I agree that surprises on the bill are unpleasant, I don't think you should have been surprised in this case.

The problem is that there was no problem with your first drink. The way your letter is written, you had a Coke and then you decided you would like to have a root beer. You didn't mention there was anything wrong with the Coke.

Therefore, you ordered two drinks, IMO. I hope the date went well.