Free 24-Hour Hold on Tickets Reserved in App
I'm not sure if this is in all markets - but booking RT travel from the US to NL using he Android app, there is an opportunity to hold the reservation and price for 24 hours without paying a fee.
In practice, the hold seems to be for a little more than 24 hours.
There is no similar free option booking on the KLM website.
Perhaps this is old news, but i was not aware.