Hi Lizanne
To answer your questions:
1. It generally doesn't matter if it isn't stamped, although preferable if it is. If possible, take it back to the post office and ask the staff member to stamp it on the left hand side in the box where the ZA letters are. A stamp is not required for it to be exchanged in the US.
Taken from the United States Postal Service website:
http://pe.usps.com/text/Imm/immc3_007.html
b. Under Universal Postal Union's regulations member countries are not required to place a control stamp or postmark on the international reply coupons that they sell. Therefore some foreign issue reply coupons which are tendered for redemption may bear the name of the issuing country (generally in French) rather than the optional control stamp or postmark. Such coupons are exchangeable for U.S. postage as specified in 372.3b.
2. Looking at Josh's feedback on this forum (
http://www.fanmail.biz/feedback/Josh_Duhamel.html), it looks like the successes from him have come from the Gersh Agency address. I think you will have a better chance of reply if you write c/o his agent.
3. 1 IRC covers postage of the minimum cost of a letter sent via air mail to anywhere in the world. Generally, you'll need 2 IRC's if a larger photo is sent (such as an 8x10 inch). If you're having trouble getting a second IRC, by all means send 1 IRC with your request. It's better than sending none, and celebrities have been known to "top up" the postage to the required amount.
Good luck!
nowandagain [rockon]
I've always used aerograms. You really helped me nicely, thank you! I'll post it Monday and when I get something back, I'll definitly post it here! 
