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