I am not an expert on Universal, but we are going in June so I have begun to do some research. My understanding is that if you get either the Preferred or Premier AP, you get 10% off additional ticket purchases. If you get the multi-day passes, I believe you have to use them within 14 days of the first use-- I don' t think there is a no expiration add-on option like Disney.
Hope this helps, and if someone knows more, I am sure they will come along soon!!
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