Required Coupon Codes + Validate Coupon

I am using Documents Seller to pass purchased music / albums to end users. They've already made their payments via a 3rd party website and I need to simply pass the music to the users 1 time.

I have set up multiple coupon codes that can each only be used 1 time so that a user cannot send the 'download code' to a bunch of friends.

I made a minor adjustment to the default.php file found in ComDMS > Views > Checkout > TMPL so that a coupon code is Required*

<?php
if ($this->config->enable_coupon) {
?>
<tr>
<td class="title_cell">
<?php echo JText::_('Download Code'); ?><span class="required">*</span>

</td>
<td class="field_cell">
<input type="text" class="inputbox" name="coupon_code" value="<?php echo $this->couponCode ; ?>" size="25" />
<?php
if ($this->couponError) {
?>
<span class="error"><?php echo JText::_('The download code you entered is invalid or expired, please try again.'); ?></span>


However, I also need the system to check that the coupon code entered actually exists. If it does they should receive the download, if it doesn't exist (or if it's expired / used already) it should give us an error.

What do I need to do?