Family controls let you pay for someone else's storage (if they're on your family account). Buying then an iTunes gift card works as well.
I prepay my $36/year in iCloud storage for all of our synced family photos by purchasing a gift card each year and then applying it to my iTunes account (instead of dinging my CC $3/month, which is just annoying).
Apple sends me an email invoice every month for the storage. If I still have a credit, it says "stored credit" was applied. I put an annual reminder in Google Calendar to re-up the account balance with a gift card. If I forget, I get the American Express push notification that Apple charged my card.
