Like many electronic sites, HotDoodle does not generate invoices. Either the funds are there when the service is rendered, or else the service is not rendered.
As a comparison, consider any of the photo purchase sites (e.g. BigStockPhoto.com or ClipArt.com). With these, they do not first give you photos and then invoice you. They simply will not give you the photos unless the funds are already available. Similarly, Google ads quit running when the funds are consumed.
HotDoodle will, however, give you a detailed accounting of how your funds have been moved or consumed. Click on "view history" for any fund and you will see each transaction.