I am looking to book a flight soon, and in some cases I am seeing lower fares through online booking sites (Priceline, JustFly, etc) than from the airline itself. (Note that these are not opaque fares - they are for a specific flight on a specific airline.)
It's rather important to me to be able to select a seat at the time of booking. This would normally be possible if I were to book directly through the airline (potentially with a fee, which I am willing to pay). If there aren't any available seats that I like, I may even want to look for a different flight.
If I book through one of these third-party sites, in general, would I be able to choose a seat at booking or immediately thereafter? Or will I probably just get one assigned to me, without the ability to change it?
It might be possible to use my confirmation / ticket number on the airline's site to select a seat shortly after booking, but I am not sure whether this always works. I understand this could depend on the specific site and airline, but is there any general practice?