Can I pay up front for my iCloud space?

You can buy iTunes Gift Card and redeem them to your account. Your iCloud Storage charge will be taken out of the gift card balance on your account each month.

iCloud storage is a monthly subscription. So every month when your bill comes do you will need to ensure you have funds present on your AppleID.

App Store & iTunes Gift Cards by Email

Gift cards redeem

Gift Card - can’t redeem

I don't know of any other means to do so. Call Apple and see if they can come up with an alternative.