You will most likely be asked to pay

If you didn't include enough postage you will either get a slip through your door saying that you have to pay more postage or reccieve a RTS.

Some celebrities have paid for extra postage. And usually post offices will put a slip in your mailbox telling you you need to come and pay extra before you can have your envelope. But, I'm confused..did you even include a SASE (self addressed stamped envelope)? If not if your talking about the envelope going to her, if there's not enough postage it will just come back to you, if your address is the return address. But, you said you sent it in October so I'm assuming you meant the SASE..?? I'm sorry if this isn't making sense. lol
Edit: I know some people use uncanceled stamps and have had it work, but I've never tried that.. I just figure it could be a bad idea..

i sent demi lovato a letter in around october, but i just ripped off so uncancelled stamps off of another autographs envelope, I love in australia so i dont know if i included enough though.i are there any success that she had paid extra for any envelopes? help!!