Walmart continues to offer the 3M Filtrete Allergen Defense HVAC Filter 3-Pack for $15.88. Opt for in-store pickup to dodge the $5.99 shipping fee. That's tied with our mention from a week ago and the lowest price we could find by $6. These filters feature an 800 MPR and last up to three months. They're available in select sizes from 16" x 25" x 1" to 20" x 30" x 1".