Nothing in the post. Take your e-ticket to the check-in and they'll be able to bring up your reservation/ticket details from that. I'd double check all the information is correct on your 'manage my booking' page, and check-in online 24 hours in advance. xx
Should only need a passport for travelling now. Last trip I took with American I had option to download boarding passes to my phone, they don't even need printing now. I saw a few people at airport scanning their phones. I didn't try it but might next time I have the option now I know its becoming popular.