I tried to send an invoice to add packaging, but the buyer paid immediately before I could do so. What are my options?

[yipeyia]

After the auction ended & I was about to send the invoice, adding in the cost for the mailing box. I tried to preview my invoice before sending & a message from Ebay came up saying it could not happen at this time. I tried to go back to the invoice to just send it anyway, but it would not let me. It went back to the selling/sold page & the buyer had paid already within the space af a few minutes. I have sent a message to the buyer asking for them to include the extra packaging cost into the total, but have not heard back as yet. What is the best way to go about this?