Default payment method question

My question is a little confusing. I am currently paying $0.99 a month for 50 GB of iCloud storage with a credit card, if I want to pay for anything else, for example, a game, how do I avoid using the credit card to pay for what I need? Am I still able to use an iTunes gift card to pay? The problem is I still want to pay for the iCloud storage, but I only want to use the credit card to pay for the storage while I can use gift cards and such at my leisure without using the credit card. My fear is that I will pay for something but it will automatically use the credit card instead of the iTunes gift card. I know its a little confusing to read through but bear with me here.