But a few things to note: Only attachment files, not item data or notes, count against your storage quota. You can't control which attachment files Zotero syncs with the server, so purging is a not a good way to manage your storage. If you don't need file syncing, you should just disable it in Zotero preferences. If you want to sync your files and you've exceeded 300 MB, you can either upgrade to a larger storage plan or set up a WebDAV server.
Upgrade Storage
But a few things to note: Only attachment files, not item data or notes, count against your storage quota. You can't control which attachment files Zotero syncs with the server, so purging is a not a good way to manage your storage. If you don't need file syncing, you should just disable it in Zotero preferences. If you want to sync your files and you've exceeded 300 MB, you can either upgrade to a larger storage plan or set up a WebDAV server.