Urban Hat€monger ?

almost but not quite.

You need to have the dated postmark on the same document you wrote the lyrics on. Putting it on a separate envelope proves nothing because you're going to need to show the lyrics, once it's opened (Which you will need to do if anything happens) there's no proof that your lyrics were sent in that envelope.
But even then it's still contestable because you could have sent a blank piece of paper and dated that & wrote the lyrics on it at a later date.
So both ways have downfalls.

If your serious about people stealing your sh*t get a lawyer to tell you what you need to do properly and don't rely on hearsay from a bunch of strangers on a forum.

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