First Data is the biggest competitor. Its 6-million-customer base generates 74 billion transactions a year, or 2,300 a second. First Data is also huge in transaction processing, for both merchants and banks: It checks accounts for fraud, ensures that there’s sufficient cash to cover transactions, approves or declines them, and sends statements on behalf of banks. First Data handles processing for 42% of all card transactions in the U.S., involving hundreds of millions of cards; its blizzard of statements makes it one of the U.S. Postal Service’s biggest customers. All told, First Data processes a staggering $1.7 trillion in payments a year in the U.S., the equivalent of 10% of GDP.

http://fortune.com/2015/10/21/first-data-ipo/