[Tagging] a "letter box" for books

[Carl von Einem]

Hi,

I'm looking for some ideas how to tag a feature I saw at a local 
library: just outside the entrance it has a box that can be used to 
return books and other media like CDs or DVDs, no matter if the library 
is still open or not. It looks like a huge letter box, no receipt returned.

That service is available 24/7 which clearly exceeds the opening_times 
of the library.

I think it's something like vending=parcel_mail_in but I'm not sure what 
tag I should use for this service.

Supermarkets often have machines to automagically take bottles and 
return a voucher: https://en.wikipedia.org/wiki/Reverse_vending_machine

And it's definitely no amenity=public_bookcase.

Carl