total = 4000000000000000 Rationalize[5.50] + 2 Rationalize[2.86];
AccountingForm[N[total, 20], {\[Infinity], 2}]
tax = total Rationalize[0.0765];
AccountingForm[N[tax, 20], {\[Infinity], 2}]
AccountingForm[N[total + tax, 20], {\[Infinity], 2}]