If you felt it is the good deal, get hold of the serial number and check with apple.com support before paying.
After paying, since it is a USPS mail, you can place a hold with your local PO
and pick up the mail, right after signing out, get the package open in the PO facility, and check the inside content. Should there be problems talk to PO guys, this is what I can think of to get a smooth transaction.