If you sold it on the 18th you had plenty of time to open the non-payer case, wait 4 days and then close it so that you got your credit before the end of the month (in the same month that the fee was applied). 

Even if your billing month ends on the 30th of the month, you get a few days to pay your fees.  If you think you're not likely to sell anything else for a while to use the credit you'll get in your account next month (if you close the non-payment case), if you don't pay the bill this month it'll show as zero at the end of next month because of the credit (assuming you close the non-payment case after 4 days). 

If you think you can just close your account and not pay because you've got nothing in your paypal or bank account, it doesn't work that way.  You can't close your ebay account while money is still owing, and even if you don't have money in your bank account ebay will chase the money using debt collectors.  You really do need to close the non-payment case to get that credit for your fees!