You should mark the DVD received normally, and use the mini-survey portion of the "DVD received" page to make a note about the postage amount due.
The sender will get credit for sending the DVD automatically, but she or he should compensate you for the missing postage.
You can arrange any solution that you like (the sender can give the credit back, or send you postage, or another DVD for no credits...whatever you agree on).
Related Links:
How much does it cost to mail a DVD?
How do I contact another member?
Getting Around the new SwapaDVD--where IS everything?