I would like to offer Free shipping at the cart and checkout when a specific code is entered.
The way in which you can achieve this is by creating a Free shipping code under the tab
Promotions>Shopping Cart Price Rules inside this option you will be able to create the free shipping coupon. Here are a couple of screenshots to show how the coupon code configuration should look to achieve free shipping:
In Matrixrates it is not possible to set a specific delivery method from the CSV as a free shipping option but it is possible to display a free method instead. To change a specific shipping method to free shipping you’ll need our Premium Rates extension.
“Free Shipping Promotions” will need to be set to “Enabled”. The “Free Shipping Display Text” is what will be displayed on the frontend when a promo rule is used in the cart.
By default Matrixrates will display the free method in addition to the other methods returned from the CSV. To display only the free method set “Only display free shipping” to “Yes”.