This question comes from this accepted answer and is supposed to clarify this answer, which states that the airline is getting paid for the flight, which is booked via third party, when you check in.
However there was a discussion in comments whether this applies to all segments, and all airlines on the ticket, or only for the flights one checked in.
So the question:
If I book a round-trip flight through a third party, with forward leg on KLM and return leg on Delta, at which points KLM and Delta are being paid?
Again, this answer says "at check-in" but it is unclear whether both airlines are paid on first check-in, or each airline is being paid at each check-in.
Applied to travel, this is important to understand whether you should check in for a return leg on a throwaway ticket or not.