SORRY I MEANT TO ADD THIS IN THE SUPPORT OR BUG SECTION _ PLEASE MOVE
When a coupon code is added there is a green success message saying "Discount Coupon Successfully added"
This also shows when the coupon code is invalid
I tested this by switching OPC off and running a valid & invalid code and there you see a success or error message, but as above always shows successful with OPC.
Ok. i tried to check without Vmuikit one page. But your staging site returns ssl error when i checkout.
But i guess problem is in awe coupon. see the ajax request after click coupon button. it returns same message even i tried with wrong coupon,