With orders of $25 or more, Amazon offers Prime members the Rust-Oleum NeverWet Liquid Repelling Treatment Spray Kit, model no. 274232, for $6.83 with free shipping. (Non-members can spend $49 or more to qualify for free shipping.) That's tied with yesterday's expired mention (although that wasn't an add-on) as the lowest total price we've seen. (It's the best deal today by $2, although it goes for $15 or more in most retail stores.) The kit covers up to 10 sq. ft. and protects a variety of surfaces from moisture, damage, and corrosion.
Note: This item is part of Amazon's "add-on program" and can only be purchased with an order of $25 or more.
Update: It's currently on backorder with an expected ship time of one to two months.