Space Available Mail (SAM) refers to parcels mailed to APO/FPO addresses at parcel post rates first transported domestically by surface, then to overseas destinations by air on a space available basis. The maximum weight and size limits are 15 pounds and 60 inches in length and girth combined. From overseas locations, items mailed at Standard Post rates are sent to CONUS by air on a space available basis. The maximum weight and size limits are 70 pounds and 130 inches in length and girth combined. All classes of mail addressed to FPO addresses must contain the New Navy Standardized Address format, which includes ship or mobile unit number, (or PSC number for ashore FPOs), virtual mail box number, and five-digit ZIP code to ensure delivery. For more information, contact your local civilian or military post office.
For more on shipping items, see this Military.com page.