There may be a need to generate an invoice for service/setup charges. In order to do so, click on the "Customer Invoices" (ref. 1.) tab and then click on "Add" (ref. 3.) to create a new invoice.
Specify the customer (ref. 1.) and the due date.
Click and drag across "VoIP Credits" (ref. 2.) to the left panel to add it as a line item on the invoice.
Change the description (ref. 3.) to match the reason for the once-off charge.
Change the value of the total "$" (ref. 4.) amount for that line item.
Finally, click "Create". Once created, there will be a button on the right to email the invoice to the customer. The customer is now able to see this as an open invoice in their customer portal.