Hi Jonke,
(Question: "So which [charity] do you usually choose for Amazon Smile?" Linked answer: "Haven't seen FSF or FSFE on Amazon Germany yet..."; "I also looked for them in vain".)
The FSFE has an associate ID with Amazon, but it's not actively used. This is the first I've heard of Amazon Smile, and I believe it's relatively recently available in Germany (as in the last year or so).
I would suggest you ask our office manager, Ulrike, to look into it further.