Free Shipping not being selected

I've got free shipping for orders of $100 or more. If the buyer does not have enough in his cart, when he starts checkout it only shows my flat rate shipping charge. That's fine. But if he stops his checkout, puts more product in his cart and then returns to checkout, the free shipping option will appear along with the flat rate charge, but flat rate remains selected and checkout is not recognizing he has enough for free shipping, i.e. free shipping option is not selected.

I want free shipping option to be selected by default in this situation. How do I do that?

For Free Shipping over a given amount, use the setting on the Modules ... Order Totals ... Shipping ot_shipping ...

This will then be the only shipping option ...

If you want to offer Free Shipping as an alternative method of shipping for orders over a given amount then use the Free Shipping Options freeoptions shipping module and set the values for where Free Shipping should show at the same time that the other shipping modules display ...