If you do not want to pay for storage then cancel the subscription.
This support article explains how to cancel a subscription from Apple on an Apple mobile device, Mac, PC, and online (see the Android section) ➔ Cancel a subscription from Apple - Apple Support
Notes:
• You might need to scroll down to find the Cancel Subscription button.
• If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew. An "Expires" date (instead of "Renews") means the current subscription has been canceled but will work until the expiration date. "Resubscribe" (or Renew) is just in case you change your mind about having it end.
• Deleting an app does not cancel a subscription.
• “If you signed up for a free or discounted trial subscription and you don't want to renew it, cancel it at least 24 hours before the trial ends.” Exceptions in: About canceling subscriptions in Germany, Israel, South Africa, or Türkiye - Apple Support
• Free trials will usually automatically turn into paid subscriptions unless you cancel in time.
• If you don't have an Apple device, see the options near the bottom of the support document.
• Many subscriptions are set to renew automatically until you intentionally cancel them.
As for the payment, you likely have others set as fallbacks:
From this support article --> How apps, content, and subscriptions from Apple are billed - Apple Support
"Which payment method is charged?
When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:
1. Apple attempts to use your Apple Account balance to cover the full amount. This is at the top of the list always, but isn’t what is referred to as the “primary” payment method.
2. If you don't have Apple Account balance, you don't have enough to cover the full amount (balance will be used first, up to whatever it can cover), or an item cannot be paid for with Apple Account balance, Apple instead attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the non-balance list in Settings on your iPhone. You can control which payment method is at the top of the list. [See: Add a payment method to your Apple Account - Apple Support ]
Don't confuse having to have a payment method present as an indicator it will be used. Apple may require a payment method be present even if it doesn't use it.
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." See also sections B and H of Legal - Apple Media Services - Apple