Currency exchange rates on flights

I’m writing this on a plane flight ( it’s ok, flight mode is on ) after hearing the flight crew offer currency exchange rates on the plane during the flight for purchase of items directly from their in-flight shop.

The currency exchange rates are horrendous!

The best advice I can offer here is to make sure you have local currency in cash before you board any flight from any country as the airline will always charge you a lot more than the standard currency exchange rate on a plane.