Join Date: Sep 2005
Location: HEL
Programs: AY Platinum, TK Elite, BT VIP, AA, BA, SK, DL, NT, WB + hotels
Posts: 8,737
If you have a Finnair flight number and your AA EXP card is in the booking, you are able to select seats online free of charge. I had a case where the AY site tried to charge me for a European flight. This glitch was fixed by their web support, so if this is your case, just give them a call. If you are lucky, you get a really good agent. Call +358 9 818 1611.
However, if you have an AA flight number, you might be out of luck. Some travellers have reported that they have succeeded in selecting seats on AY's homepage for AA-flight numbers, but it doesn't seem to work all the time. A phone call might or might not help.
Basic rule number one for me is: never codeshare, no matter what the marketing department tries to make us believe. It's always a hassle of some sort.