"Box bookstores" refers to large retail stores that primarily sell books, often located in warehouse-like buildings. They are typically known for their wide selection of books and discounted prices.
I've run into this with other big box bookstores around the country. (madgeniusclub.com)
Note: Don't get discouraged if big box bookstores turn you away; many are restricted by corporate policies from helping authors outside of publisher - arranged collaborations. (selfpublishingrelief.com)