Hi, this gets asked alot, here is a topic which covers this just a few post under your own:

Click here

Otherwise why not wait until he is in the US and you can do return postage much easier?

Hello everyone,
My first post cause I need some help. I want to send a few items to Andrew Lincoln in London, UK. I called the post office and the IRC were discontinued in January this year. How do I include a SASE without an international reply coupon? Do you put US stamps on it or send US money to be used as return postage? I really want to do this soon before Mr. Lincoln comes to the US to start taping the 4th season of the Walking Dead. Any information would be greatly appreciated.

Thank you
Bryan