You don't need a license, if you are just taking uCommerce for a test drive or are developing a site.
uCommerce will run with all features enabled if you install it locally. Or if your domain name contains words like stage, staging, preview, demo, developer etc. So you can easily create a local project and work on it, and show it off to your client using a special domain name (like projectx.staging.mycompany.com).
Even if the above conditions are not met, uCommerce will continue to run. It will just show a "Developer Edition" banner at the top of your website, to notify you that this is the time to get you a license. But it will still be fully functional.