Hmm, the trade in credit may be part of the issue.  The other is that if this was done at one of the mall kiosks, it may have been a reseller, and not a corporate store.  They often have their own terms and deals... The trade in credit may be superseding the buy one get one half off deal, and so the second credit for 1/2 off is not getting applied ... this may need to be escalated through customer service, which I can't do.  I'm just another customer like you. 

yarelim_vzw​, can you take this further?