Rather than being performed in the shopping cart, upgrading to a paid hosted service is actually done in Account Management. 

Do the following on convert your trial:

1. Log into the SmarterTools Portal at http://www.smartertools.com/portal
2. Log in using your account information in the upper left (if you are not already logged in)
3. Click on the Account Management tab
4. Click on Hosted Services and Trials
5. Click on your Hosted Service
6. Click on the Subscribe Now button and follow the steps provided