enigma1 - 8:35 am on Jul 13, 2010 (gmt 0)
My advice would be, when you get emails like this to first get information from the customer about the way they got the problem and see if you can replicate it. Redeem a coupon may work for you with the browser and configuration you setup. But it may not work for somebody else because he uses something else.
Lots of eshops these days work exclusively with flash or js or with a specific browser. I shop online with these things blocked many times and if I cannot enter or use the cart of the store I don't even bother to email the store owner. I just continue to the next store.
If get feedback like that, I will get as much info from the client as possible and try to replicate the problem. All the carts that I have seen have many side effects many times difficult to identify.
All I am saying is don't rule out the possibility the customer couldn't redeem the coupon.