Your cash balance is found in your Billing Summary (see blue box in image below).

If you upgrade, downgrade, remove numbers etc., you might have a cash balance. This is because we now prorate accounts. To see past information, click on View History. The Cash Balance History table will appear. If you have a history, you will see the transactions.

You can use your cash balance for any one-time payment when manually applied. For example:

For more information about billing, check out this helpdesk article.