With orders of $25 or more, Amazon offers Prime members the WaterPik Power Flosser, model no. FLA-220, for $5.24 with free shipping. (Non-members must spend $35 to receive free shipping.) That's $2 under our October mention and the lowest total price we've seen. (It's a current low by $7.) This battery powered flosser comes with 15 tips.
Note that this item is part of Amazon's "Add-on program", and can only be purchased with an order of $25 or more. Also, note that colors may vary.