Nextel buyback program

I cancelled my service with Nextel in March. I was told they had a buyback program and was mailed a mailer with instructions. Both I710 phones were placed in their mailer and dropped in a mailbox per instructions. I have been waiting since then for a check or a bill with a credit showing. I have talked to about 12 different people at Sprint/Nextel today and all say they can't help without a tracking #. Since I was instructed to just drop in a mailbox, I have no tracking #. What can I do now?