Free shipping code display issue

I have setup a free shipping code for orders over certain amount. Now I have three shipping options: parcel, first class and priority.

When setting up the free shipping code, under Bonuses I clicked on the “Add Bonus” three times and picked “free shipping” for each of the three shipping methods I have configured.

The problem is that when customer enters the code at checkout the three shipping options are all shown each with a radio button to choose from!

I would like the customers to just see “free shipping” displayed after using the code and no shipping options with radio buttons to choose from, even if all their rates are correctly set at zero.

Right now they see this:

USPS First-Class Mail (2-5 days) - Free shipping

USPS Priority Mail (2-3 day delivery) - Free shipping

USPS Parcel Post (2-9 days) - Free shipping

How did you handle similar scenarios? Can someone help?