The simple check is: If the flight doesn't go, will any payment or benefit still be made. If the answer is No then it is clearly for the purpose of the flight!
If you fly me to Le Touquet, I might choose to buy you lunch in Le Touquet. If we don't fly, I won't buy you lunch, because I can't. By your test, that makes the buying of lunch remuneration for the flight.
I suggest reversing the test. If the payment is not made, would the flight still go? If I don't agree to buy you lunch, will you still fly me to Le Touquet? If the answer is no, then the buying of lunch really is remuneration for the flight.