First Alert CO605 Plug-In Carbon Monoxide Detector w/ Battery Backup $18 + Free S&H w/ Walmart+ or $35+

Walmart [walmart.com] has First Alert CO605 Plug-In Carbon Monoxide Detector w/ Battery Backup for $17.99. Shipping is free w/ Walmart+ (free trial available [walmart.com]) or on orders $35+

Amazon [amazon.com] also has First Alert CO605 Plug-In Carbon Monoxide Detector w/ Battery Backup for $17.99. Shipping is free w/ Prime or on orders $35+ (Thank you, Citan359!)