What to do if you sign up for something but did't pay?
In order to get the payment window again you will need to cancel your current sign-up & sign up again. Go back to the calendar listing, scroll to the bottom where you see your sign-up & click Edit to the left of your name. Change the status to canceled & then click to save.
Then start the sign-up process over. Once you submit your calendar sign-up the payment window will pop up for you to make your payment.
This will need to be done before sign-ups close. All unpaid sign-ups will be deleted after a sign-up closes.