With orders of $25 or more, Amazon offers Prime members the Oral-B Glide Pro-Health Comfort Plus Floss 43.7-Yard Box 2-Pack in Mint for $3.72. Clip the $1.00 off coupon on the product page to cut the price to $2.72. Plus, Prime members also bag free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's at least $3 less than you'd pay at a local retail store, although in January we saw it for a buck less.
Note that this item is offered as part of Amazon's "add-on program" and can only be purchased with an order of $25 or more.