The Smart Battery Case is one of the most practical iPhone battery cases, but its price is also one of the most exaggerated. So when its price is halved, it's a good plan. The Smart Battery Case for iPhone X / XS is “only” 75 € on Amazon Germany with shipping costs to France, instead of 149 €, therefore.
You can connect to Amazon.de with your usual Amazon credentials, the only difficulty will be to remember your German lessons to order (although the path is well marked by Amazon with its yellow buttons).
Apple's battery case roughly doubles the battery life of the iPhone X / XS. It can be charged wirelessly or with a Lightning cable.