This plugin also includes an IPN (Instant Payment Notification) endpoint that will update your WHMCS invoice status.
Initially the WHMCS invoice will be in a Unpaid status when it is initially created.
After the invoice is paid by the user, it will change to a Payment Pending status.
When BitcartCC finalizes the transaction, it will change to a Paid status, and your order will be safe to ship, allow access to downloadable products, etc.