Is it Flixbus still? I thought everything was MeinFernBus now. Anyway, their attendants load the bags in the hold as you hand it to them. So it will be based on their "guestimate". Also, I thought that a third bag for the hold incurred the extra charge, but it is not something they guarantee to accomodate. That it's all based on the available hold capacity after the other passengers' bags are loaded.