Costco.com sells 2 (2) packs of Winix 5500 or Winix C535 replacement filters for $89.99 (plus tax in your state) with free shipping. You get 2 Hepa filters and 2 carbon filters. You don't have to be Costco member for this online purchase, and there is no non-member surcharge or markup.
2 (2) packs of 5500 or C535 filters for $89.99
Walmart and Amazon are selling same filters, in a single pack for $52.52. This includes 1 Hepa filter and 1 carbon filter.
Double down on the savings for filters @ Costco for $89.99.
Or buy a whole new Winix C535 purifier with filters for $99.99 after coupon and includes free shipping. This new unit comes with 2 Hepa filters and 8 carbon filters. This is the biggest bang for your buck. Once again, you don't have to be a Costco member to buy a new C535 @ Costco.com.
New C535 purifier with filters for $99.99
These go fast so get em while the gettin is good.