There are a few options here,
you could pursue a warranty claim through ATT, but they will make you pay shipping for the charger to send back and it will be along drawn out process (they are known for poor customer service), or you could even buy a new charger on your own (I can help with the parts),
but the best thing you could do would be to simply return the entire system to costco and ask them to replace/exchange the system for you.
If you have the option to, I'd highly recommend exchanging it for a panasonic unit instead, they are much better quality and it will serve you better in the long run.
If you have any other questions, just let me know. Otherwise, please take a moment to rate my service as this is the only way I am credited for my time. Hopefully I have earned 5 Stars but if not please let me know what I can do to ensure you are satisfied. I also invite you to contact me directly with any future support needs at http://bit.ly/1QwLxGh and ask for Nathan.
Thank you, ***** ***** a nice day,