Walmart offers the Universal Double Window Business Envelope 500-Pack in White for $20.05. (Amazon has it for the same for Prime members only with free shipping.) Opt for in-store pickup to avoid the $5.99 shipping charge. That's the lowest price we could find by $7, outside of the mention below.
A close price: MyOfficeInnovations via eBay has it for $21.32 with free shipping.