Hi,
Apologies for this. It's an error in the system. That amount is actually the Google Checkout fee. It shows you the right amount when you go to make the payment but when it tells our system it's paid, it doesn't take into account the fee. So it came up with a negative figure. We're fixing the bug just now. Don't worry, in the next few hours you should see it fixed completely.
Apologies again for the confusion!
-Tungro