Posso pubblicare una lettera di 20 g come questa? (L’ultimo 5C è al di fuori del “Frankierzone”)

If it’s within size and weight limit. Sure.
There have been times, it was considered to be a fun message from family members on vacation to pave poststamps on to a letter like wallpapers.

The DeutschePost may have not been happy about it, but all these letters arrived.

Edit: Make sure to write a return address on the back.

I’ve sent stuff like this before and had no problem. Maybe the automatic scanners might not be sure about it, but they still use people to take care of what the machines struggle with.

I’d say your chances are good this’ll be fine.