本日は都合により、午後六時で閉店___。
Q. Which humble form announces 'we will (be allowed to) close'?
Translation
Due to circumstances, we will be closing at 6 p.m. today.
Explanation
させていただく frames one's action as permitted by the other party — standard in announcements: 閉店させていただきます. なさる is honorific for others' actions, and いたされます is malformed.