You might not want to do this. The shipping from the US to me for the proof copy is anywhere between $6 and $20, depending on how quickly I want it (fortunately, as it's my first book, they give me free shipping). The cheaper options can take 4-8 weeks for delivery. So if a signature is worth either $40 (both ways) and 2 weeks waiting, or $12 and 2-4 months of waiting, then... uh, I don't know... I've vastly underestimated how much I can ask? :-)
Conversely though, if he sends you the book without return postage, he might not get it back.
