Although there isn't a way for you to change your storefront URL yourself through the website, just shoot us an email and let us know what you'd like it to be and we'll get it taken care of.
When emailing us, remember that the URL needs to be in the following format: http://YourCustomValue.instaproofs.com
Our email address is: support@instaproofs.com
Our support hours are from 9am to 5pm MDT, Monday through Friday.