WP e-Commerce, how to process $0.00 sales and 100% coupons which PayPal will not accept

The Problem

I had a question recently about why a coupon was not being processed at the PayPal end. It turns out that PayPal will not process a $0.00 sale. If you have an item for $10.00 and a discount coupon for $10.00 the coupon is processed on the WP e-Commerce checkout page showing a total of $0.00, but when the user gets to PayPal the coupon is totally ignored and the price is $10.00. Not good!
If you reduce that coupon to $9.99 the sale would go through with a total of $0.01. That is not a very elegant solution. Many sites now have partnerships with coupon sale sites, these third party sites sell coupons which the user then expects to use to purchase at no cost. Some people have tried using the Test Gateway but this is only going to work if all of your sales are $0.00. You still want to send customers who have a cash balance to pay on to PayPal when they click Purchase.

Solution

Thanks to getShopped user dhtbrowne who supplied this solution http://getshopped.org/forums/topic/how-to-stop-free-products-redirecting-to-gateway/#post-268526

I will reproduce his code just as he wrote it and add a little explanation as things can be hard to find on the getShopped forums and even have a habit of disappearing completely. I have tested this with Storefront Themes, Elegance theme version 1.4.5, WordPress 3.3.1 and WP e-Commerce 3.8.7.6.2. It should work for most themes and hopefully newer WP e-Commerce versions. Please leave a comment if you test it in higher WP e-Commerce versions.
The following solution will intercept any $0 purchase and the “wpsc_purchlog_edit_status()” changes the order status to payment accepted, sends out all the necessary emails and deactivates coupons if they are set to use once.

STEP 1.
First you need to make a page that says thank you, for users to be redirected to after they click purchase. Get the Page ID of the thank you page. To do this open it in WordPress for editing and look in the URL bar of your browser you will see a URL ending in something like this.
wp-admin/post.php?post=492&action=edit
You need that number, in my case 492, it is the ID of the thank you page.

STEP 2.
Here you need to edit a PHP file it is very easy, just pasting in some code, so you can avoid the whole code editor thing if you want and do this in WordPress. Go to Appearance/Editor, find functions.php on the file list and open it. Right BEFORE the very last ?> paste this code and change the 492 to the ID of your thank you page. When done make sure you have no blank space or lines after that final ?>

	
add_action('wpsc_submit_checkout', 'free_coupon_test');
function free_coupon_test($order_info) {
global $wpdb;
$purchaseID = $order_info['purchase_log_id'];
$rows = $wpdb->get_results("SELECT discount_data, totalprice FROM " . $wpdb->prefix . "wpsc_purchase_logs WHERE id=$purchaseID LIMIT 1");
require_once(ABSPATH.'/wp-content/plugins/wp-e-commerce/wpsc-admin/ajax-and-init.php');
if ($rows[0]->totalprice == 0 && !$rows[0]->discount_data == '') {
wpsc_purchlog_edit_status($purchaseID, '3');
header("Location: " . get_permalink(492));
die();
}
}

Remember change the 492 to your thank you pages ID!

Thats it, if you have done it right any $0 purchase should redirect to your thank you page. You should receive a purchase log and the customer receives a thank you receipt. To check it, try buying something yourself for $0 so that you get both emails.