How do storage add-ons work?

Pro includes 500 MB of encrypted sync storage. If you need more, add-on packages stack on top: 1 GB for $4.80 per year, 2 GB for $8.40, or 5 GB for $18, up to 5.5 GB in total. Add-ons are the only recurring purchase in the product, because storage is the only thing that costs us money every month you use it.

Everything is managed under Settings > Storage: switch to a bigger package (you pay only the prorated difference) or cancel anytime and keep the space until the period you paid for ends. Pro itself is yours forever either way.