Amazon has the TP-Link HS200 Smart Wi-Fi Light Switch the cheapest I've seen for a single pack - $19.49 after the $3 clipped coupon. There have been comparable price-per-unit FP deals for the two pack (i.e. two for $37 or two for $40). But if you only need one, then here you go.
Because the cost of one is less than $25, free shipping is only available for Prime members or if you add filler items for those that don't have Prime.
I think clipped coupons can be YMMV sometimes (and may not appear if you've used the coupon already). Simply going into incognito mode shows the coupon still there, but I have no other way of checking universal qualification.