I want to buy more storage. How do I pay for it?

How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance or you don't have enough to cover the full amount, Apple attempts to charge your primary payment method. The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.

Various ways to pay for items in the iTunes and App Stores.

- Add money to your Apple ID balance - "With the payment method that you have on file, you can add funds to the Apple ID that you use for iTunes & App Store and iCloud Storage purchases."

https://support.apple.com/HT208687

- Payment methods that you can use with your Apple ID - https://support.apple.com/HT202631

then

Change, add, or remove Apple ID payment methods - https://support.apple.com/HT201266

- Create and add money to an iTunes Pass - "At Apple Stores and select retailers, you can add store credit directly to the Apple ID that you use for App Store, iTunes Store, and iBooks Store purchases." - https://support.apple.com/HT203021

- Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209