Daniel Radcliffe only sends preprints through the mail (via his agent). This means it's a copy. If you're ok with this, go ahead and write, otherwise consider waiting for him to appear on stage again.

Your envelope does not need to be special, though you will need to include a stamped return envelope.

I am sending a letter to Daniel Radcliffe and I requested a autograph and asked him to send the picture
Do I have to send a picture for him to sign it?
Do I need a special envelope for my letter?
This is my first sending off autograph My dad got me 2 books and 2 signs from Brian Jacques as he visited my dads work place before he went usa.

Thanks in advance