You can send a full Series Volume to print early, even if you don't have a credit available. You'll be charged the same price as a credit, which depends on the size of your Series.
How do I send a Series book to print without a credit?
Tap into your Series subscription
Tap a Volume that shows "Ready to print"
Tap "Send to print" at the bottom
If you don't have a credit, tap "Continue to purchase"
You'll see the price before you buy. To use a future credit instead, tap "Wait for my next credit."
Review your order details, then tap Place order
Learn more about how Series credits work in How does a Series subscription work? (Series Only)