The shuttle bus back to Itzehoe runs very regularly on Sunday until everyone is gone. On my first trip my friends and I took our sweet time leaving and decided to wait for a taxi, but the shuttle bus staff yelled at everyone to get on the bus instead. Like they didn't even make us pay for a ticket; they just wanted us to f off so they could go home.
As a general rule, don't expect anything to be open on Sundays in Germany apart from essential services like food, transport (the drivers, not the station staff in regional areas), select pharmacies, etc. If you need to send anything home and it can't wait until Monday, do it on Saturday.