They actually tested buying DVD's and electronic chargers and they successfully cheated the payments systems. But of course they returned the items after the tests has been done. These are the following ways they cheated the system:
1. Add discounts of their choosing
2. Shop for free after paying for one item
3. Purchase an expensive item for the price of the cheapest itemThey said that the problem is with the third party services not that payment systems itself. Wang explained the logic of the bugs with a naughty child who wants a piece of candy scenario: "The child tells the mother that the father said it was OK, and then tells Dad that Mom said it was fine. If the two parents aren't communicating effectively, the kid gets the candy."
The said researchers reported the problems to the merchants and closely worked with them to fix the flaws or bugs in the systems. I guess they already fixed the problem so don't bother trying. Read more about this in CNN.