Hi Jonah,
I've checked and the airline will charge if you would like to book a seat at this stage, so you'd need to log onto the website and pay online.
However, they will be free of charge once you're able to check in which is 48 hours before departure.
I'd be happy to send you your reference number by email if you would like to pay in advance, please let me know.
Many thanks,
Vickie