To enable the Simple Shipping add-on, you'll go to Exchange > Add-ons and click the Disabled button to the right of the add-on.
Once enabled, you have settings you can adjust. To access your Simple Shipping settings, you can either click the gear symbol to the right of the add-on or go to Exchange > Settings and select the Shipping tab at the top of the page.
In the settings, you'll find General and Simple Shipping settings.
Under the General settings, you have the option to make the global settings changeable for each individual product and determine whether or not to exclude non-shippable products from the per-order price calculations.
Setting this option to Yes will show an Override option when you're creating a product. If set to No, you will not see the Override option when in the product screen.
Under the Simple Shipping settings, you can set the price for your Flat Rate Shipping and determine which shipping options will be available (Flat Rate Shipping and Free Shipping).
Here, you can also change the text for your shipping options and set the global rate for the Flat Rate Shipping option. You can also change the Flat Rate Shipping price on a per product basis.