Amazon offers its Prime members the Filtrete MPR 300 Clean Living Basic Dust Filter 6-Pack for $20.39 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $7.53.) That's tied with our mention from two weeks ago and the lowest total price we've seen. (It's also $3 under the best price we could find for this quantity of comparable Filtrete filters elsewhere.) It comes in sizes 10x20" to 25x25".