Nothing. Granite for Shopify is free, and every feature works whatever Granite plan your account has: every product, changes within seconds, images stored in the vault, metaobjects, discounts, navigation, translations, the files index, subscription plans, and write-back to Shopify.
The store's notes live in your Granite vault, so they count toward your Granite storage like any other notes. If the vault fills up, syncing pauses and everything already synced stays readable.