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How do I offer free shipping on orders over a certain amount?

You could select either "free shipping" or "Non-free shipping" mode for your store as explained in

When you select "free shipping", the shipping fee will be included in the product selling price and customers do not have to pay for additional shipping fee. There is no need to give them free shipping again for order over a certain amount.

When you select "Non-free shipping", the system will calculate the shipping fee based on the weight of the entire order. In the "Non-free shipping" mode, there is no room for discount in the shipping fee as we have to pay the same shipping fee to the shipping companies for shipping out your order. If you would like to offer customer an incentive for them to order more products on your web store, what you could do is you could provide a discount coupon to them with minimum order of $100.  Please refer to this article how you could set up the discount coupon: