You should mark the CD received normally, and use the mini-survey portion of the "CD received" page to make a note about the postage amount due.
The sender will get credit for sending the CD automatically, but she or he should compensate you for the missing postage.
You can arrange any solution that you like (the sender can buddy you a credit, send money through PayPal, contact us to transfer money to your account, or send a free CD)- whatever you both agree upon.
You can direct the sender to the Help Center item "How do I give credits to other members?" for instructions on how to send you a credit, or send them this link:
http://www.swapacd.com/help/help_item.php?id=103
Related Links:
How much does it cost to mail a CD?
How do I contact another member?
Getting Around SwapaCD--where IS everything?