So now you offer free shipping for a minimum purchase amount. That's great. There's just one problem: Your customers don't know. They don't know because you haven't told them!
When you enable free shipping, your customer won't see it as a shipping option until he/she is checking out. So free shipping might come as a nice surprise to someone who has met the minimum purchase. But it won't encourage the customer to add more to their order unless you find a way to tell the customer.
One place where you can advertise free shipping is in the shopping cart side block that appears on every catalog page. In the following screenshot, you can see the side block labeled MY CART. Normally, this block displays only the number of items and the total. But we have added a message to the block: We offer free shipping on orders over $100! Does your order qualify?
Now let's have a look at customizing the default theme. Your store's layout, terminology...