Originally Posted by FlightDetective

Is it definitely asking you for payment? I booked only a few days ago with Avios, DUB-HEL on a 320 and HEL-LHR on the 350 and it lets me choose seats just fine on Finnair's site with no cost.

It looks like it's going to charge you, but when you select everything it's still zero.

My only advice is maybe to wait until tomorrow in case something sorts itself out overnight, but you shouldn't have to pay as a BA Silver (oneworld Sapphire) - as Finnair give their Gold (Sapphire) and above free seat selection so it should be the same.