I would like to upgrade my icloud storage but I don't have a credit card? What other ways of paying are there and how do I submit to them?

I bought an Iphone 6s and tried to upgrade the iCloud storage. The gadget inevitably leads me to the question of my credit card data to pay for the upgrade. I don't have a credit card because I never need one and I don't want to get one just for this purpose. Are there any other ways of paying for the upgrade, e.g. by billing me for a year in advance, and how can I transmit my wish to my iPhone?