Please make a place where you can add a billable items (service from the site (i.e. hosting account) or extra (just typed in). Then some options for later billing, such as:
1) Do nothing (Leave it as a "draft" for later editing)
2) Add to the user's next invoice (this item will then be added to the next invoice generated for the user)
3) Bill on next Billing Cron (This item will then be billed on the next billing run)
4) Bill now (Create and send the invoice now)
These are single line items that can have the above choice for each, so if you created 10 different billable items and wanted 3 of them to billed now, you could set 2 of them to add to user's next invoice and 1 to bill now. Equally, if the user's next invoice is at the end of the month, you can set an item to "Add to user's next invoice" and it would be added at the end of the month with the user's next invoice.
This is useful for irregular items and off off items that need to be billed at a later date but do not fit the need of a draft invoice type.