Here's what I'd do. Go to the store when things are quiet - like right after opening on a weekday. Pick out some CDs you were going to buy anyway. After you pay, ask for the manager, make sure no other customers can overhear, and politely explain that you buy all your CDs there (you do, right?) and you would really like that poster when they are done with it. Explain that you don't want him/her to go to any trouble, you'll come in whenever it's convenient to pick it up and what is the most convenient way to handle it for him/her.
If the manager starts to blow you off, just ask quietly if you could pay $10 for his trouble for getting you the poster. If the answer is still no, I guess you're sunk.
But you'll have a few things going in your favor: you're holding a bag of stuff you just bought, so you're obviously a customer; you made sure no other customers overheard which would put the manager in an awkward situation; and you volunteered to make it as easy as possible.