Adding your storefront to the Infiniverse Directory is free, but might not remain so in the future. Click Add Listing on the front page and follow the instructions.
Adding your commercial storefront to the Infinimaps Business Map (http://infinimaps.com) comes with a minor fee. Contact us at info@infinimaps.com for current rates. Non-commercial storefronts can be added to the Non-profit storefront map for free, also by getting in touch with us on info@infinimaps.com.