Dear Bitter Amanda,
I think I received a note from Dr. Who. Can you please confirm?
I assume you mean Doctor Who, as one doesn't abbreviate when referring to The Doctor.
So, are you a future companion? Unlikely. And not just because I'm fairly certain The Doctor wouldn't leave notes on old receipts. (I'm not saying he has some fancy embossed stationary or anything--I really wouldn't know--but it's just a hunch that this note is not he he rolls.) I say no mostly because for you to be a future companion, the author of this note would have to mean what he said and follow up on it. We all know that's rather unlikely for a man, hmm?
If you believe in magic and want to save the universe, then by all means pen a missive in reply. Just don't get your hopes up that the TARDIS will show up on your front lawn. The guy didn't even provide a return address. Typical.