Payment of iCloud

I know when you downgrade a storage subscription you receive an email telling you there is a 30 day grace period before data will be purged to get your used storage down to below your subscription allocation.

But I don't know if that also applies to defaulting on your payment. You can always contact Apple iCloud support directly to find out what the policy is -> https://getsupport.apple.com/?caller=home&PRKEYS=

Not sure how long a grace period is extended but in the meantime you could purchase an iTunes gift card and load the balance to your account to pay for iCloud storage.