Do you know PayU? It is a Paypal competitor on emerging markets. A few weeks ago I was talking with my friend, who was working on online payment integration with PayU REST API. He showed me that you can play around with payment details and PayU API won’t complain about amount not mathematically correct. It is possible, that client will increase products quantity without increasing the amount to pay. I invested some time to test all the cases and this article will show you how to hack PayU payments and to get more than you paid for.