It could be that at some point in the past, you scheduled and paid for an exam and then canceled that exam. At SmarterProctoring, we don't refund the money once you have paid for an exam. The money is then credited to your account for use toward another exam. See the example below:
On April 14, 2016, Johnnie schedules a Biology exam that is a 2 hour exam. For this exam, Johnnie pays $25. Then Johnnie cancels the exam due to an illness. A $25 credit is then applied to Johnnie's account for the Biology exam that he canceled. Then, on April 20th, 2016, Johnnie schedules a History exam that is only 1.5 hours and would normally cost $20. The History exam will appear to Johnnie as being free. This leaves a $5 credit on Johnnie's account. Then, on May 7th, 2016, Johnnie schedules a Math exam that is a 2 hour exam and would normally cost $25. Johnnie will be charged $20 for the Math exam after his $5 credit is used.