Hello everyone. Apologies if this is the wrong sub-forum to post this into.
I'm trying to book one way flights from Athens to NYC for 2 people in July. There is a non-stop flight pricing to $900 USD total (operated by Delta) when I look on the Air France site or the KML site. When I try finding the same flight on Delta.com, its pricing to 1700 Euros total.

If I go ahead and try to book on KLM or Air France, I get a message stating that I am unable to select seats - even if I try to go for the highest level of Economy. Any idea what I can do about this? I spoke with Air France and they essentially said any changes to the ticket need to be done with Delta. Can I buy the ticket through Air France/KLM and then call Delta to do seat selection? Worried that Delta will not allow me to do this.