Downgraded to Basic -but still have a monthly cost?

I sold the last of my land on December 9th, and chose to downgrade my account to Basic. The downgrade successfully occurred on December 13th. However, at the bottom of my account information I see this:

Peak square meter usage:512 square meters
Total monthly cost:US$5.00

I emailed billing six days ago and received no response whatsoever.
(Ticket #515807)
I telephoned you (international call) at dead-on 9am (your time) today, but got stuck in a telephone queue for ten minutes before hanging up.

You pay tier according to the peak amount of land you held during the previous billing period...not necessarily the month. Your billing period may start in the middle of a month. So if you owned more than 512sqm in the 30 days previous to downgrading, even for an hour, you will be billed for it.

If you hadn't owned more than 512 in that period and you are being billed, there is a problem and you will need to contact customer support. You can email [email]billing@secondlife.com[/email] but calling is your quickest option. Please be patient should there be a queue as CS has been very busy lately.