I'm looking at fares to Barcelona end of the month. The AA website is generally pricing at $1,800 or more RT from SFO, but looking at some other vendors (Priceline, Expedia, Travelocity), I can see some routings/segments on AA metal under the Finnair codeshare flight number at $400 less, using the exact same AA flights that are selling for $1,800. This would mean booking outside AA.com. I'm trying to think of the disadvantages in doing this, and the only one that jumps out is having the deal with the vendor, rather than AA, for issues like seating. As I have not been through the booking process, it's not clear to me if you have an opportunity to enter an AAdvantage number to even open up better seats. AA.com indicates I should get full miles and elite bonuses on Finnair flights. What am I missing? Any hazards in booking the Finnair flights outside of AA.com or in booking the codeshares?