There is nearly a decade of precedent for complaint-free use of the amzn.com domain here on HN[0].
The amzn.com link I provided is as much an Amazon link as your expanded version (both domains are owned by Amazon[1]).
I choose not to use the official, shortest a.co option (growing in popularity here[2][3]) because it obscures a tiny bit more context (the Amazon Standard Identification Number is usually recognizable as such).
In this case Amazon promises the shorter url IS a permalink, it does not make the claim that the longer link will be permanent to the best of my knowledge, although it's a good guess they will also be retained.
