Actually, you don't even need to create a Coupon for that.
You can simply change your
US Shipping Profile to
$0.00 US and all your customers from US will get Free Shipping during the Checkout.
To highlight that this is a limited time offer, add a line like "
FREE Shipping to US until ...." under "General Shipping Policy" on the
Shipping Profiles page. That message will appear on every product page below your Shipping charges.
I've noticed that you have only 1 country listed under Shipping section. This might be a reason why you are not getting much sales or product enquiries, as you are limiting yourself to customers from US only.