Figures obtained by The Herald under freedom of information legislation reveal Lothian Buses raked in an additional £194,035 above the value of ticket sales in 2012, equivalent to an extra £530 per day.
The bus company, which is more than 90%-owned by Edinburgh City Council with the remainder owned by East Lothian and Midlothian councils, operates an exact fare policy. Drivers do not carry spare cash, so passengers buying a £1.50 single ticket would not be able to get change if they only had, for example, two pound coins.
Public transport campaigners criticised the exact fare policy - which is also used by private bus firm First - as an "out of date embarrassment".
The surplus for Lothian Buses has increased from £142,882 in 2011 - around £391 per day - but figures for 2013 will not be ready until May.
The figures do not include the value of foreign and counterfeit coins received, which amounted to almost £95,000 in 2011 and more than £68,000 in 2012. Green MSP Patrick Harvie, who spearheads the Better Buses campaign, said: "The exact fare system is an out of date embarrassment that runs counter to the notion of 21st-century public transport.
"The discovery that bus firms, even those as well run as Lothian Buses, are making an unfair profit from passengers should prompt an urgent overhaul. A proper integrated, cashless ticketing system like London's Oyster card would be ideal but we're still miles behind in that journey."
John McCormick, chair of the Scottish Association for Public Transport, said passengers needed smartcards which would allow them to switch between buses and trains, as well as the tram in Edinburgh and subway in Glasgow.
He said: "The requirement for cash payments for fares on Scottish public transport is an entirely outmoded system. Transport operators in advanced parts of the world use stored-value cards, such as the Oyster card in London.
"This dramatically speeds up the operation of buses as passengers board much more quickly, without having to queue to pay the driver. This also eliminates the 'overpayment' problem."
But Lothian Buses stress the surplus in 2012 accounted for just 0.2% of its total turnover and that the discrepancy between fares collected and tickets sold was not always caused by overpayments.
For example, on occasions where an on-board ticket machine was not working passengers would still pay fares without being issued with a ticket.
Exact fare policies are also seen by some operators as an important way of reducing assaults against drivers, by enabling to place drivers behind safety screens which mean there is also no way for drivers and passengers to exchange money directly.
A spokesman for the Confederation of Passenger Transport Scotland, which represents bus operators, said: "To allow for driver safety and quick boarding many bus operators choose to use cash hoppers to collect fares.
"Overpayment reclaim policies are also often in place for passengers without the exact fare.
"As smart and digital ticketing options grow this will mitigate any overpayment issue. However, many passengers value the ability to pay in cash and the cash hopper system remains a safe and efficient method to allow for this."
Lothian Buses said passengers could request overpayment slips from the driver, which can be reclaimed within five working days at the company's Hanover Street Travelshop.
Ian Craig, chief executive of Lothian Buses, said: "The exact fare system, which we have operated for over 40 years in line with many other UK cities, means less delay for passengers when boarding our buses and helps us to keep our services to time. We recognise people will not always have the exact fare available, which is why we encourage passengers to use our flexible and convenient ticket options, including our new Smartphone m-tickets."