Supply and demand. The department stores' can deliver, digitec cannot.

Because that is the price of the rip-off gang from daswarenhaus.ch. Unfortunately, Digitec tolerates extortionate prices in its own shop from third-party suppliers. The Digitec price is lower: Nikon Z Teleconverter TC-2,0x (Teleconverter, Nikon Z)
Actually, the vendor is clearly visible but it seems that no one pays attention.