OP Updated

If you reach the total volume of 1,000 USB sticks, price is dropped to 0.89 each. We will also pay back the price difference of all your historic transaction (timeline started from the 0.99 pricing) of USB sticks. For example, if you bought more than 1,000 sticks, 500 for 1.99 and 500 for 0.99, the new price will be 0.89, and we will pay you the price difference for the 0.99 USB sticks, that is (0.99-0.89)*500=50BTC.

Sorry for the original ambiguation before that misled some customers that the price difference we pay back is 1.0BTC. What we expected to say is for the price difference based on both new prices.