I credited 300 to my icloud account,may i know how to use them
How to subscribe by using credit in my icloud
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Just subscribe to whatever you want. Apple ID balance is automatically used to pay for anything you buy or subscribe to. You don't need to do anything special to use it.
Do note it may still require to verify your payment method before using the balance, but won't charge your payment method unless there is no enough balance to cover the cost of the item or subscription. If all is correct, it will automatically use the balance in your account for any purchases or subscriptions you initiate including iCloud storage.