Groupon offers the Purafilter Gold Air Filters 4-Pack in 4 sizes for $19.99 plus $3.99 for shipping. (Note that more sizes are available for $1, $2, $3, or $5 more.) That's tied with our mention from last week, $36 off list, and the lowest total price we've seen. Each filter purportedly lasts up to 90 days. Deal ends February 28.