The delivery cost can be set in a stagering model so that it would be free over a certain order value. This also makes it very clear from the customer perspective and by setting a suitable limit it may attract customers to place bigger orders.
The delivery method must be created as the "Exemption limit" type. Create the delivery method on the Settings > Delivery and payment > Delivery methods page by adding the name of the method and choosing the "Type" as "Exemption limit" from the drop down menu.
Once you have created the delivery method you set the standard cost of the delivery in the "Base price" field on the Settings page. Set the "Free above an order value of" field with the limit above which the customer does not have to pay the delivery cost. You can also choose if you want the order value to be considered before or after potential discounts have been applied.
In the shopping basket there will be a reminder for the customer about how much more would need to be added to the basket in order to be elidgible for free delivery.