Most B&M won't accept online coupons, from what I've read. There are ones especially for B&M and they often get picky when you give a code without proof of where it comes from. Again, from what I know you shouldn't spread coupon codes around, so you're lucky you can just show a bunch of them, because you can get blacklisted (if you do it online or over the phone).
Either the manager at your store is very nice or the clerk is a newbie.
I can be wrong, so let others have a go at it.
P.S. Where's the overnight shipping that should be added to the PM?