I think it would be easier to do an OMS order. OMS, is an order completed in-store that can be sent to any Best Buy store for pickup or sent to your home with free shipping. When you do an OMS order, they allow you to use Gamer magazine coupons.
Just ask an employee at the Gaming counter if they can do an OMS for the game, if they don't know how to do one, you can usually go over to the Customer Service or Store Pickup areas to do it.
For what it's worth I've done exactly what you are proposing, bought online with ship to store, went for pickup, asked for them to cancel and let me buy it w/ the coupon. But I've only done that once and don't know if it's YMMV.
Thanks for the info... what does OMS stand for? I'd like to sound like I sort of know what I'm talking about, lol. If bananafish had success doing it the way I suggested, I will probably just do that. I find the people at my best buy to be a little bit dense sometimes (actually I find that in most retail stores), so its best not to confuse them. I will often order things for store pickup, cancel, then buy it with credit I just got from trading in. Sounds like this will work pretty similar.