<form name="paymentform" method="post" action="{$SECUREURL}/cart/process/{$token}/">
<div class="box contact-form">
    <h2>Pay by credit card</h2>
    <input type="hidden" name="token" id="token" value="{$token}" />
    <input type="hidden" name="handler" value="dps_pxpay" />
    <p>Click the button below to be redirected to our secure payment provider.</p>
</div>
<div style="text-align: center;">
  <input type="submit" name="pay" id="pay" value="Pay by Credit card" onclick="if (true){ldelim}$('#pay').attr('disabled',true);paymentform.submit();{rdelim}else{ldelim}return false;{rdelim}" />
</div>

</form>

<form name="paymentform" method="post" action="{$SECUREURL}/cart/process/{$token}/">
<div class="box contact-form">
    <h2>Pay by credit card</h2>
    <input type="hidden" name="token" id="token" value="{$token}" />
    <input type="hidden" name="handler" value="dps_pxpay" />
    <p>Click the button below to be redirected to our secure payment provider.</p>
</div>
<div style="text-align: center;">
  <input type="submit"  class="button" name="pay" id="pay" value="Pay by Credit card" onclick="if (true){ldelim}$('#pay').attr('disabled',true);paymentform.submit();{rdelim}else{ldelim}return false;{rdelim}" />
</div>

</form>

Add button class to the submit buttion
Add button class to the submit buttion
