S.C. Code § 7-15-230
Upon receipt of the ballot or ballots the absentee ballot applicant shall mark each ballot on which he wishes to vote, fold it so that its contents cannot be distinguished without unfolding it and mail it in compliance with the instructions received by him with the ballot. Enclosed with the ballot or ballots shall be the oath set forth in Section 7-15-220. No ballot shall be counted unless the oath is properly signed and enclosed therewith nor shall any ballot be counted which is received by the board of voter registration and elections or other officials charged with the conduct of the election after time for closing of the polls, and the printed instructions required by item (3) of Section 7-15-200 to be sent each absentee ballot applicant shall notify him that his vote will not be counted in either of these events.
S.C. Code § 7-15-230