If I have a provisional document that is listing the items that will be billed for this month (subject to change, because the month is not yet finished), then what is the better term? "draft invoice" or "upcoming invoice"? Or something else?
Neither. It could be a
proforma invoice (Business Dictionary)
An abridged or estimated invoice sent by a seller to a buyer in advance of a shipment or delivery of goods. ... Pro forma invoices are commonly used as preliminary invoices with a quotation, or for customs purposes in importation. They differ from a normal invoice in not being a demand or request for payment.
The document is a form of unbilled account. It can't be any form of invoice because the invoice is presented only when the billing period comes to an end and the customer may well order more goods before that time. It's not a proforma invoice because the goods have been delivered and a proforma invoice is a document presented prior to delivery showing, for the customer's information, details of the cost when the goods have been delivered.
You will continue to add items to the unbilled account until the billing point at which time you will use the information in the unbilled account to create an invoice applying any discounts and calculating any tax due (if your government expects you to collect such a tax on their behalf). You will then present the invoice to the customer. After that point the invoice becomes part of your accounts receivable and the unbilled account can be moved to a record of invoiced items known as your billed account.