Every VPS is metered hourly from your account credit balance. The public
monthly figure is a 730-hour equivalent so you can compare plans; a 31-day
month bills 744 hours.
The hourly rate is derived from the advertised month and rounded down, so
you are never billed more than that month over 730 hours.
Bandwidth overage and Client API overage (after the included monthly
allowance) also debit credit. The invoice list in the portal is the receipt
history for deposits and those charges.
Credit is a ledger. Staff adjustments and deposits append rows; there is no
balance field to overwrite.