I'm new to autograph collecting through via-venue, and would appreciate some help.

Firstly, I'd like to send an autograph request via-venue to a concert. I've checked with my local postal service and it takes 7-12 days for an item to be shipped via airmail to the US. How many days in advance from the date of the concert should I send the mail?
Secondly, since I live ouside the US, I understand that I'll need to include an IRC or stamps in my SASE for the letter to be posted back to me. Is it alright if I just include an IRC in my SASE, or is it better to get US stamps? My concern with stamps is that I may not get the postage rates right and might end up with the letter not being posted back to me.
Thirdly, I'm intending to include a poster from a magazine to request for it to be signed. How can I ensure that the poster gets back to be in good condition (without being folded and bent by the postman)? Is it sufficient to write "Please Do Not Bend" on the envelop?
Thank you so much!