STOP PRESS: This contribution is now updated to Version 5 and has all sorts of cool extras. Please see the latest post about this.
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I've been slowly but steadily working on this contribution when I can find time in my schedule to do so 
Seems as though there are a lot of requests but hardly anyone is willing to help further this modification by donating to the project…
V4 – Admin Side:
Able to add a "use once" or "use multiple" voucher.
Able to add a "fixed amount" or "percent discount".
Each Coupon shows the date/time of the last use (linked to the order) and the total number of uses.
This link enables you to quickly see every use of each voucher.
This list shows each use of the voucher. Each use is linked directly to the actual order made.
You can disable coupons easily by clicking these buttons.
Version 4 – Shop Side:
The Coupon Input box is now in the shopping cart page. This means that other contributions such as "FEC" should work OK.
Inserting a valid coupon code changes the value of the cart contents appropriately.
The "remove coupon" link does just that – it removes the coupon from the order.
The checkout_confirmation.php page shows the details of the order as usual, including the coupon.
And the same information is shown in the admin page for the order.
V4 – under the hood
Version 4 has had quite a lot of code changes – mainly to deal with the fact that coupon discounts can now be placed outside the checkout procedure.
V5 – in the pipeline
Minimum Spend per coupon.
A "use once, one time" coupon. Once that is done, I can then look at the flip side of discount coupons, which is "gift vouchers.
