Nionic charges a monthly fee per court (250 MXN per court by default),
billed on the 1st for the previous month, counting the highest number of
courts you had that month. Your first partial month is free.
Pay by card from the Billing section. While an invoice is open you see a
banner; from day 15 you can't create new events; with two or more open
invoices your players can't reserve courts. Paying everything open clears the
block immediately.