I Returned My Defective Phone But Am Worried That I Will Get Charged.. Help!

Hi. I got my replacement phone yesterday and packaged my original phone up and sent it back today. I did put the provided return label on the box with the phone inside but I am worried that it will get lost somewhere and that I will get charged. If this does indeed happen, then how can I remove the charge since I did send the phone back. I have an RMA # as well as an Order # and a Tracking # that got emailed to me. I would like to be assured that I will not be charged for a new phone if the phone I am returning ends up getting lost or stuck in transit. An AT&T agent told me that they can track the IMEI number to see if it indeed got lost and that I may or may not get refunded for the amount of the new phone. Please help me on this, thanks!