When the USPS attempts to deliver a package to cluster mailboxes in an apartment or condo complex and the package is too large to fit into the recipient's mailbox, the carrier will use a parcel locker instead.
From the USPS website:
Cluster Box units often have parcel lockers. The parcel locker is used for items that are too large to fit in an individual box (compartment) within the Cluster Box. A key will be left in the mailbox to be used as a parcel locker key. This key has a tag that indicates the parcel locker that contains the item. Insert the key into the correct parcel locker, and the mail piece can be retrieved. The key will remain in the locker once it is inserted.
If you are unable to locate your parcel locker or are unsure how to access it to retrieve your package, please check with your apartment complex or condo office for assistance.