If you dont pay ahead, they could be booked or you'd have to wait a while. I recommend booking ahead. Check Costco, they have discounted gift cards there (ex. $50 iFLY credit for $40).
Book ahead of time. Access is appointment based.
It's usually good to check availability to make sure there are free slots before hand and possibly reserve a slot. Then you can pay on site. I recommend that once your done, you take advantage of the extreme discount for pre-buying the next session then.
It is also cheaper if you go on groupon
You can do it either way. Usually cheaper done ahead
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