tell them your situation and give them your paypal transaction reference - you should be able to get this from logging into your paypal account and searching your purchase history.
They should be able to confirm000. that for you and reply that they have found it. Then print off their reply and take that to the festival with your print off of your paypal payment history and ID.
Now I dont work for Primaverasound but I am pretty sure this is the way to go to get everything sorted