It sounds like you have an older version of eCart.
the most recent version of eCart is 6.1.0. this sounds like a bug that was in a previous version that has since been corrected.
uninstall the previous eCart version from the Extension Manager, then install the newer one that is available for download in your account.
When you open DW, it should launch the WebAssist installer, if that does not launch automatically, open a php page to force it to start.
After the WebAssist installer completes, make sure to restart DW.
Then delete the existing checkout pages and run the checkout wizard again to recreate them.