If you return the initial purchase completely and then you buy something else separately then you may have to contact Amex if they take the credit back (one time use promo so I doubt the system will auto apply it again). If you're doing an even exchange then it shouldnt matter -- I think BB processes the return & the new item in a single transaction and it'll break even (I dont shop at BB, but I think thats how they do it)
If there's a chance that you may want to return an item or buy something else then you're better off just buying a $100 GC first and then using that on your purchase (if you return the item in the future then they'll just give the amount back on a GC)
What happened was I bought an e-gift card, then saw it was through a third party processor. I called AMEX to see if it was a problem, and was told it probably wouldn't count. I then begged for the gift card people to cancel/return the e-gift, and they did. Two minutes after, I get the email showing that it apparently was a qualifying purchase.
They never applied the credit, and I bought a normal 100 dollar gift card to ship. That one went through fine. I'm just wondering if I need to call em again, because Amex CSRs really have no clue about the promotion, I've found...