"This booking does not exist"
Tried to book a flight 7 times.. Every time I would come to the "Pay" button, it would tell me that the booking no longer existed. And every time it would add 20/30 euros to the price. Eventually I said "bye" (in other words), and booked directly via the airline. It was even cheaper.. :)
I've seen this bad user experience in other websites, and it usually happens when you don't choose any of their extras (like flight cancellation security, extra seatspace etc). Bad UX = no booking, so it doesn't pay off for Flysmarter in the end.