If you would like to send a Series Subscription book to print without an available credit, here's how:
Go to your Active Series
If the Series Volume has 60 pages in it- go into the Volume, and tap "Send to print" that is located at the bottom of the screen- it will look similar to this screen:
Once you click "Send to print," you'll see the prompt that asks "Want to send your book to print early?" that looks similar to this screen: