A rollershutter is a percentage value for openHAB internally. Put aside that stop was wrongly implemented by me and a fix is on the way (after everybody returned from holidays), UP and DOWN will send the target percentage. This might not be clear from the docu (create an Issue if so).
If your shelly wants “stop”, “up” and “down” you should define a switch with those three commands instead. (Can only be done textual on item level unfortunately)