There are three squadrons in the 315AW. They are the 300AS, 317AS, and 701AS.
If you are not sure which squadron the person is in, you can always send it to the 315th Operations Group and ask them to forward it (they will know which squadron he/she is in).
The street address is preferred, but I bet you can just send it to
NAME
c/o UNIT (e.g. 300AS or 315OG)
CHARLESTON AFB, SC 29404
You can also call the base operator at (843) 963-1110 and ask for the base locator. They can give you the unit and address of the person you are looking for.
Good Luck.