There may be a misunderstanding about how openHAB works. Item commands are usually passed to bindings e.g. KNX and on to the real world actors. Item state updates are not. Incoming data from KNX usually causes state updates in openHAB, and does not create OH commands.
There are exceptions, these are provided by KNX binding by way of the -control type channels, which work the other way round.
Your rollershutter-control example would normally be a control panel with buttons, when you push a button the KNX message gets passed as an openHAB command (so that you might control e.g. a zwave shutter or something).
To complete the backward-working effect, -control channels pass Item state updates to the KNX bus.
This might help
but really I don’t think you want to use a -control channel here at all.
The state of an openHAB rollershutter Item is 0-100 (a position percentage) and you would normally send UP or DOWN commands, although a target position as a number is also an allowed command.
Group Items have a state that is calculated by an aggregation function, like AVG in your example.
You cannot postUpdate a new state to a Group type Item.
You can send commands to a Group Item, these will get passed along to all member Items.