The problem is that there is no initial quantity so the add to cart code is not being executed fully. You will need to default the quantity to at least 1 in order for the item to be added to the cart. You can do this with a hidden form element in your form like this:
<input type="hidden" name="WA_Store_Cart_1_Quantity_Add" id="WA_Store_Cart_1_Quantity_Add" value="1" />