Walmart offers the 3M Filtrete Allergen Defense HVAC Filter 3-Pack for $17.88. Opt for in-store pickup to dodge the $5.99 shipping fee. That's the lowest price we could find for this quantity elsewhere by $9, although we saw them for $2 less in our mention from last August. These filters feature an 800 MPR and last up to three months. They're available in select sizes from 12" x 24" x 1" to 20" x 30" x 1".