If you've reached the limit of Unlisted Publications, you will not be able to add new Unlisted Publications to your account.
If you need to keep more documents as Unlisted Documents, you would need to upgrade to one of the Unlisted Publication Tiers listed on the Account Services Page.
Note: The Unlisted storage you subscribe to will renew monthly until cancelled and is not a single charge. For instance, if you select the 1500 unlisted storage package, your card will be charged $150 each month, until cancelled.
When you choose to cancel the unlisted storage, you will no longer be able to upload any new unlisted publications.