(I had the same thing happen to me when I sent my fingernail clippers to myself from an airport one time...don't ask...)
You might just tape the keychain in between two sheets of cardboard and put it into a similar envelope. This should keep the sorting machines from mangling the envelope by eliminating the sharp edges. Wrapping the keychain in 3 or 4 sheets of paper isn't good enough (again, don't ask me how I know).
PS: I got my keychain, with only a small hole punched in the letter and envelope. But I never got my nailclippers, just the envelope.