How to purchase and add iTunes card to account

Here is: What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

Here is how to buy (and redeem) an: Apple Gift Card - Apple

Gift Cards can’t be used (alone) for subscriptions and for some other purchases, as you’ll usually also need a payment card on file even if the subscription is debited from the Aple ID balance.

I’d be cautious about making gift card purchases in stores if the gift cards are openly and accessibly displayed, due to the potential for scams and fraud.