You can set free shipping for over X (where X is any amount you want) by
going to Shipping Settings in the admin section. Click on the Shipping
Zones tab and click 'Edit Settings' on the zone that you wish to
provide free shipping to. There will be an option called 'Enable Free
Shipping? '. Tick this, then enter in the minimum amount for free shipping in the Order Total to Qualify
field.
When the customer checkouts out with a purchase over X, they will see the option for free shipping. They sill have the option of paying for a different type of shipping, such as express shipping.