Trying to complete an order with a coupon offered by Stardock and isn't working

I was trying to decide if should buy or not Start11 for multi-device. As per the email below, I can use the coupon provided to get an EXTRA 15% Off and the coupon isn’t working at all. Tried in 3 different browsers (Firefox, Chrome, and Edge). I don’t get any message saying the coupon was expired or not valid. It just doesn’t apply the proper discount.

Greetings,

We noticed you had items left in your cart and have yet to complete your purchase. Come back and pick them up with an extra 15% off* with the coupon provided above. Make sure to complete your order today - we can only hold your items for a limited time.

Any help would be appreciated.

Thanks