It's a great feature if you need some extra cash, but this option will not stay there forever, it will disappear soon if you take it or not.
Paying off is like If you do order for $100, normal you would make $80 but when you have taken cash advance fiverr will deduct extra 40% from your $80 earned so you end up with $48
($32 is paid off for your cash advance automatically) It was not bad at all for me when I had it.