Sure! Ubercart has a very flexible shipping quotes system that allows you to create all sorts of shipping options for your customers.

Offering free shipping for orders over a given dollar amount is a two step process. The first step is to set up the shipping method (or methods) you want to use for orders less than or equal to your cutoff amount. The second is to set up a Flat Rate shipping method to charge $0 for orders over your cutoff.

At admin/store/settings/quotes/methods you can enable the shipping method(s) you want to use. By default, there are no Flat Rate methods shown. Go to admin/store/settings/quotes/methods/flatrate and select "Add a new shipping method" to create a Flat Rate method called something like "Free Shipping for orders over $xxx" - the exact name doesn't matter, just use something meaningful to your customers. Set both the "Base Price" and the "Default product shipping rate" to zero and press "Submit". (If you want to also charge a flat rate for orders under your cutoff, you should now select "Add a new shipping method" again to create a second flat rate with the appropriate dollar amounts.)

Now that your shipping methods are set up, you'll have to configure Workflow-ng display each quote to the user only under certain conditions: Namely, when the order total is above or below your cutoff value.

On the admin/build/workflow-ng page, you will see a list of Active Workflow-ng configurations for each shipping method you've enabled. Click on the name of the shipping method, then click on "Add a condition". You will be presented with a select box of available conditions - select "Check the order total" and press the "Add" button. Fill in the "Order total value" textfield with your cutoff amount and select the appropriate radio button - either "Total is less than or equal to specified value" or "Total is greater than specified value". Pressing the "Submit" button will apply these changes to the quote method. Return to admin/build/workflow-ng and follow these same steps for all your active shipping methods.

When your customer checks out, the shipping options that appear will now depend on the order total. If you did everything right there should only be one option, "Free Shipping", shown if the order total is greater than your cutoff!