AF tweeted me:
Hi Ticu, sorry for the delay in getting back to u. We checked with the relevant department. There is a technical issue on the website. 1/2
The seats are booked and guaranteed for you free of charge. Please ignore the payment request online. Your booking is annotated. Thanks. 2/2
How this will play out at the check-in time still to be seen. But at least I have it now in writing that I don't have to pay (the website still asks for payment at this time though).