Hi Hissoacouk40
Welcome to the SBB community, where customers help other customers.
As of right now, all reservations for PostAuto busses are not mandatory, except a few selected journeys. Here you can find all the busroutes starting from Meiringen where a reservation is mandatory. If you take any other bus, you can’t reserve a seat. (Note that these busses only travel from 28th of June onwards.
Hope that helps, best regards
Zebis