Make sure you have the most recent version of eCart (6.0.6) installed.
if you have a previous version installed, make sure to uninstall it from the extension manager before installing the the newer one.
When running the checkout wizard, on the first screen make sure that you enter the site URL in the Site address setting. this URL is used for setting the return address. If not set properly, the return will fail.
also, on the email tab of the checkout wizard, make sure to uncheck the Don't Send an Em,ail receipt option. If this option is not unchecked, the UE behavior will not be added by the checkout wizard.