Can I use an authorised representative from another EU country or must it be Germany?

It must be Germany. The Stiftung EAR explicitly requires an authorised representative based in Germany. This is because the Stiftung EAR is a German registry and authorities can conduct local inspections. An authorised representative from France, the Netherlands or Austria will not be accepted by the Stiftung EAR. This is a common misconception among international sellers trying to cut costs. However, specialised service providers in Germany offer competitive authorised representative services and handle the administrative work.