Bear has some suggestions for sending more than demanded and then some.
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If I had a "bump stock" (say, something I had no use for until being told I couldn't have one and got just to establish standing -- and that's a big "if"), my inclination along with filing a lawsuit would be to follow the #IWillNotComply paradigm.
If I were inclined to surrender my property, I might be inclined to turn it over to another government agency first, my local federal collection center, addressed to ATF (c/o 1600 Pennsylvania Ave.?) and marked "POSTAGE DUE -- CONTAINS BUMP STOCK" and let them deal with the additional uncompensated expense:
Maybe I'd even do it wearing a large hat and sunglasses, or a disguise, just in case that's frowned upon through regulations.
Then again, I like my first inclination better. If I had a bump stock...