[Wreck_C]

But safe to say my box attempt failed. I am thinking even if I send them a "tempered" empty box for <100g item, they could've easily came to the same conclusion and deny refund, wouldn't they?

I sealed box a +200 shaver with a fresh account and after 10 days i got refund, and after another week i received the same mail as yours.  On the site it looks like the refund is completed too. I deleted the card from the account and no requests of rebill for now.
I did the same thing with my original +10 years old account and they did advance refund, even if the return page says “in transit”.

Nice one I am hoping the same outcome. I deleted the vcc right after I got the AR, there's no way they can rebill me but they could, theoretically, reverse the AR transaction (according to revolut) but I am confidence that the bank would side me given that I have a valid tracking that shown items delivered 2 weeks ago.

Although they made sure that I cool off on SEing them for now by making it a pain in the ars*

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Boxing only works with Amazon if it's an item that is not checked  right away (usually not expensive). WIR only works if you have a good standing account and it's like your first SE. 

Have you done DNA or EB before in this account? or did you let them know you received wrong item?

This account boxed amz.pl once about 2 months before this first attempt on amz.de. I DNA amz.de on another account without using proxy so that could be a mistake.

This box came about after a failed DNA.

From what you are saving, it seems the AR saved it, not so much the boxing method itself. The order was about 300eu though.

Yes, advanced refund helped. They also likely disabled AR option on your account now. Basically amazon won't email you about this if it's your first attempt. When they are emailing you it's because it's not your first time abusing policy or your account been flagged from 2 many SE.