Groupon offers the Purafilter Gold Air Filters 4-Pack in 43 sizes from $17.99. Coupon code "VISA5" cuts the starting price to $17.09. With $3.99 for shipping, that's a buck under our July mention, at least $34 off list, and the lowest total price we've seen. Each filter purportedly lasts up to 90 days. Deal ends February 18.