Why am I being billed for service after canceling service?

I canceled my service with Cox at the very end of my billing cycle going into March, specifically on that date to avoid paying a partial for another month's service that I wouldn't need. I'd been with Cox for 3 years so it isn't an early cancelation fee, and I returned my router when I went into the store to cancel service. But Cox have sent me a bill for ~$39 when I did not use their service at all, after I canceled. What is this 'bill' for, exactly?