JSFiddle - React, Tailwind, and code Playground

HTML

<form id="myForm" name="PaymentForm" method="post" action="http://mysite.com/dap/paypalCoupon.php" />
<input type="hidden" name="cmd" value="_xclick"/>
<input type="hidden" name="currency_code" value="USD" />
<input type="hidden" name="item_name" value="Product1" />
<input type="hidden" name="redirect" value="http://mysite.com/go/site.php" />
<div><input type="image" src="http://sarniayachtclub.ca/sites/all/themes/syc/button_add.gif" border="0" name="submit" alt="Make payments with PayPal - its fast, free and secure!" /></div><br>
<div><strong><font color="#32424a" size="-1">Coupon Code:</font></strong>
<input type="text" name="coupon_code" size="20" /></div><br>
<input type="checkbox" name="tos" value="1" /><font size="-1">I have read and agree to the <a href="http://mysite.com/site-terms-of-service/" target="_blank">Terms of Service</a></font><br />
</form>

JavaScript

$(function(){
  //Make the button initally disabled
  $('#myForm input[type=image]').attr("disabled", "disabled");

  $("#myForm input:checkbox[name=tos]").click(function(){
    if($(this).attr('checked')){
      //Show the button and enable
      $('#myForm input[type=image]').removeAttr("disabled");

    }else{
      //Gray out the button and disable
      $('#myForm input[type=image]').attr("disabled", "disabled");
    }
  });

});