The most polite response would be to wish the person a "Shabbat shalom" in return. It's like wishing someone a nice weekend.
Shabbat Shalom means "peaceful Sabbath" and is the standard greeting between Jews on Saturday.
"Shabbat Shalom" (peaceful sabbath) is an appropriate form of "goodbye" from about late Friday morning until about Saturday afternoon. Between sundown Friday night and sundown Saturday night, it's...
E-Mail BEFORE Shabbat.
You respond the same way: Shabbat Shalom