“Global Payments Inc. agreed to buy Total System Services Inc. in a deal valued at $21.5 billion, the payment industry’s third mega-merger of the year.    The transaction will create a powerhouse that provides payment technology and software to more than 3.5 million small to mid-size merchants and more than 1,300 financial institutions worldwide, the two companies said in a statement Tuesday. The all-stock deal values Total System Services, or TSYS, at $119.86 a share, a 20% premium to its $99.62 closing price on May 23, before Bloomberg first reported deal discussions with Global Payments.”