Yes you can, simply make sure your DNS A record is set to point to your store. You can find the A record you need to use from within your Instantcart account at www.instantcart.com/account.
Once you have logged into your account, navigate to the "Stores" section, located the store you wish to setup a DNS record for, and then click on the "More" button.
Scroll down to find the A record IP address you need.
Login to your domain registrar, or DNS provider and enter the A record entry into the appropriate configuration box.
Allow 4-8 Hours, and your store domain should point to your Instantcart store.
Don't forget, you'll need to park your domain name ontop of your store, however please wait until your domain name has resolved to its new location before doing so.
To park a domain, please see our Parking a Domain Name guide here