I just tried this for DVDs, had 2 orders for 35.99 and 14.99 (Black Lagoon series 1 v3 LE, Otoboku v2), so I canceled them both and tried to place the order again with the BMLDVDSI or BMLDVDS1, whichever it is - neither worked. Both DVDs were sold from Amazon, were preorders to be released on 8/16, but I got an error that the promo code could not be applied to my order. Why? My total was 50.98, and while I did have 45.60 in gift certificates to apply, the error occurred before I was able to apply them, so that shouldn't matter... I don't get it.
Edit: Maybe it's because I have Amazon Prime? I think you're automatically not eligible if you have it - that's stupid!