I have the same issue.  I returned my 5S in December.  I have the tracking slip showing that it was received in Ft. Worth on December 19.  They can't find it at the warehouse and have billed me for "non-return of the device".  I've been on the phone for hours over the past two months but they won't credit my bill.  My local "We Are Wireless" store has called twice on my behalf and Customer Support always promises that my credit will be applied within 3-5 business days but it never happens.  I just called again today and was told I need to give the warehouse another 30 days to resolve the problem.  They're just stringing me along.