Invalid Charges For Returned Phones

Does anyone have any advice on dealing with AT&T on this issue:

-I returned two iPhones under Next to get credit for an upgrade.

-I received an email last week saying the phones were never received and they were going to charge my account $275 for the two phones.

-I checked on tracking and it shows: "The package is delayed and will not be delivered by the expected delivery date. An updated delivery date will be provided when available."  This status was last updated on January 13th.

-I called Customer Service and after dealing with several people who ran through the standard scripts, got to a person in Online Provisioning who claimed that it was all set and a credit would be promptly processed if it wasn't already.

-I reviewed my bill today and I see no credit, but of course I do see the charges.

-I contacted Customer Service today via Chat (because if I spend any more time on the phone with AT&T I think I will go insane) and they weren't able to help or fulfill my request which is simply "issue credit".

That's where I am.  

The way I see this is quite simple:  This is AT&T's return process, AT&T's selected carrier, therefore AT&T's issue.  They can plainly see just as I can that the packages are "In Transit".  

I'm also incredibly frustrated that after having been lied to on the phone since I was told clearly that this matter was  "all set and a credit would be issued" that I am in fact not "all set".

I would ask the community if anyone has experience in dealing with AT&T on this and how best to approach this?  

Since I did my part of this transaction per their specifications I am looking for AT&T to step up and just do the right thing.  I am really not interested in me having to research or file a claim with USPS or do any other work in this matter (I've already wasted a day), the way I see it they should take it from here and just issue me a credit.  

Quite frankly being a long time customer with 5 lines on AT&T Wireless I'm shocked that this situation has gone this far.