Delivering mail to cluster boxes instead of to the door or a curbside mailbox saves the Postal Service more than $11 million annually, the General Accounting Office reported. Cluster boxes are centralized mailbox groupings used in many apartment and townhouse developments and in some new residential areas. They allow letter carriers to deliver mail for several homes at one spot, saving time on their rounds. The GAO report covered fiscal 1985, when about 1.9 million of the 53 million daily city mail deliveries involved cluster boxes. According to the study, in areas of detached homes, delivering mail to a cluster box instead of to a curbside mailbox saved about $10 per customer, for an annual saving of $4.5 million.