Can you use iTunes gift cards to pay for icloud+ monthly subscription?

Yes, according to How to redeem your Apple Gift Card or App Store & iTunes gift card which states:

"What you can buy with these gift cards

When you redeem an Apple Gift Card or App Store & iTunes gift card, you can make purchases with your Apple Account balance. Those purchases can include apps, games, subscriptions like Apple Music or iCloud+, and more. But for some purchases, Apple Account balance can't be used. Learn what you can buy with your Apple Account balance."

Yes. Note that you'll need to have a different payment method on file while subscribed.

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