Why charged on a cancelled account?

That is your outstanding balance. It may have come from the removal of the auto-pay discount, since you likely cancelled before the end of the billing cycle.

To confirm this, can you provide a screenshot of your bill showing a detailed list of charges? Make sure to remove any personal details, as this is a public forum.

When you switched from Monthly to Prepaid in May, your old postpaid account would have generated a final bill with prorated charges up to the day that service ended. A 'Balance forward from your last bill' just means an amount from that final bill that's still showing as owing — here, $7.00.

Can you download the PDF bill for that former Monthly account in Self Serve and check the breakdown? That'll show exactly what the $7 is — usually a small prorated plan or Tab amount left from the final month.