Hi Georg,
I’d prefer shop=number_plate.
craft= seems not appropriate to me since the production of a plate is fully done by a machine within a minute (like amenity=vending_machine, vending=elongated_coins) and one does not need much knowledge or time to produce one.
office= seems not appropriate to me since I want to buy a thing and not a service.
shop=trophy or shop=car_parts would mix different things in my opinion. For me a number plate is not an accessory for a vehicle but a requirement. That’s why one would find an place to buy number plates next to every governmental office for car registration (at least in Germany). If at this place other things are sold too than to me that looks like a little post office in a bakery shop - same place same person but two different functions.
“number_plate” is singular, BE, and shorter than “vehicle_registration_plate” which would be my second choice.