If you have not yet paid:
Go to your Events tab and find your event.
Click Pay Invoice.
Click Edit My Sold Items.
Make modifications as necessary. You can change information about the winning bidder by clicking Yes by the item in the "Did you sell it?" column.
Click Continue Checkout and submit your payment.
If you have already completed checkout:
Go to your Events tab.
Find your event with a status of Invoice Paid.
Click View Details.
Click Add More Sold Items
Report any additional items you sold.
Click Continue Checkout and submit your payment for the new invoice.