JSFiddle - React, Tailwind, and code Playground

HTML

<script>
    function enableSending() {
        document.form.submit.disabled = !document.form.checkbox.checked;
    }
</script>

<form name="form" action="shop_paypal.php" method="post">
   <input name="cmd" type="hidden" value="_xclick" />
   <input name="no_note" type="hidden" value="1" />
   <input name="lc" type="hidden" value="UK" />
   <input name="currency_code" type="hidden" value="USD" />
   <input name="bn" type="hidden" value="PP-BuyNowBF:btn_buynow_LG.gif:NonHostedGuest" />
 <input type="hidden" name="item_name" id="item_name"value="YOUTUBE"/>
 <select name="amount" id="amount" style="width:256px; height:32px;">
       <option value="18" id="1">10,000 PINS - $18 </option>
       <option value="35" id="2">20,000 PINS - $35</option>
       <option value="75" id="3">30,000 PINS - $75</option>
       <option value="140" id="4">50,000 PINS - $140</option>
  </select>
  <label>Your Facebook fan page URL: </label><br />
      <input class="input-box" type="text" />
      <input type="text" />
      <input type="checkbox" class="checkbox" name="checkbox" id="checkbox" onclick= "enableSending();"/> 
    <label for="checkbox" class="agree">I agree to</label> <a href="#">Terms & Conditions</a>
    <input type="button" src="images/products/buynow.gif" class="submit"  name = "submit" value="Send" disabled="disabled"/>