If I understand your question correctly, you are worried that a rejected payment will cause you to lose all of your iCloud-stored data within 15 days.
Instead of prepaying for something whose price drops on a regular basis (which might not even be possible), I would advise you to purchase iTunes gift cards. If you search around, you can find some deals on them from time to time (eg. $5 off a $50 card). That way, you protect your iCloud data against rejected payments, while keeping the ability to take advantage of the occasional cloud-storage price drop.
Your iTunes balance will always be used before your credit card (at least in the US). Load up your account with $150, and you’ve got yourself a year of prepaid 1TB storage on iCloud, with enough leftover for an iTunes Match subscription.
The risk, of course, is you might start spending more on iTunes and apps since you’ve got a balance there, but that is entirely up to you.
Note: App Store cards are equivalent to iTunes cards, but NOT Apple Store cards. Apple Store cards can ONLY be used at an Apple Store or the Apple Online Store.
A good way to think about it is:
iTunes/App Store cards = software & services, whereas
Apple Store cards = physical goods.