Shipping Coupon Zeroes Out Tax and Shipping and then reports Coupon not found

I am using AWO Coupon PRO in VM 1.1.8 and Joomla 1.5.23.
I use Polished Geek's One Page Checkout.

I have a shopper group that I would like to set up for free shipping.
I am not certain that I have things correctly set because the documentation here is sparse.

Function Type: Coupon
Function Type2: Shipping

PerCent: Per Cent
Value: 100

Shopper Group and ID show up.

There is nothing in the Shipping box. What should be there?

The result I get when I enter the coupon code is that the screen returns a message of "Coupon code not found. Please try again."

It then sets Shipping to Zero and Tax to Zero.

Clicking Update totals puts the shipping and tax values back.

What should I do?

Thanks.

Shipping options should be in the shipping box. For example if you are using ups, then you will see the ups shipping options. Not sure about this error as I have not seen it before. If you send me a private message with temp admin/ftp access I can take a look at it.