A supermarket like Rewe is an odd choice because supermarkets rarely have expensive goods but I like the idea and why not have "cold storage" near frozen goods.

Not necessarily. At least in my country that's not the case. Big chains often have a tech department, and you can buy various stuff there. Everything from chargers, cables, USBs to audio & video tech, vacuum cleaners, fridges, etc.
This is a good option if you can purchase BitBox with cash, since you won't leave any transaction trace, you won't have to add your name, email, delivery address and there is zero shipping fees.
I am certain you can. Germans will hold on to their right to use cash as long as they can. They are not pushovers like some other nations. Yes, I am looking at you Scandinavia. FFS you used to be vikings.