You can always verify how much you’re paying for a subscription credit by reviewing your Subscription Settings.
For Series books in the Chatbooks app:
Open the Chatbooks app
Tap into your Series Subscription
Tap the Settings (gear icon) in the upper-right corner
At the top of the screen, you’ll see the cost of your next credit
This amount reflects what you’ll be charged for your upcoming book credit. You can then use this credit to send a photo book to print.