There always need to be exceptions, even at QSLing. The main rule is never ask for money. But what if you are living in a poor country and have your radio build from junk parts or obtained a radio as a gift? Well that is a exception. But remember even in a "poor" country there are rich people who can easily pay for a card and postage. If you talk for example to a station in Central Africa and the operator is transmitting with a Kenwood TS590 or a Icom IC-7800 and a big 3 element commercial beam you can count that such a station is able to pay for postage!
Another exception is asking for money if the DXpedition or activation is for a purpose like support a school or a project to save people. Various examples can be thought of, like the activations in coörperation with the Mercy Ships organisation (TY1MS, 5L2MS and 9L5MS). In these cases most of the DXpeditions are donating the money (and equipment) to the project. Confirmation is also done via LOTW so you don't have to pay money if you don't want to and still get your QSO confirmed.
Remember there is also something like free QSL via eQSL or LOTW. The only thing you need is a internet connection. That could be a problem if you have a very remote location, but if so at least a log can be send to someone that can manage such a account.
Of course it's everyone's own free choice to pay for a QSL or not! It's always your own decision what a exception is. But think....before you pay for a QSL that could also be for free.