If you want formulas to accumulate over the full report, then you need to assign it to a global variable. Then you can print the global variable at the end of the report. Here is a sample formula that you can play with:
Global NumberVar Summary;
Summary := Summary + ({TradeNetMessage.TotCustomDutyAmt} + {TradeNetMessage.TotGST} +
{TradeNetMessage.TotAmtPaid} + {TradeNetMessage.TotCustomDutyAmt})
To print the summary, create another formula which simply returns the value and put that on the report.
Global NumberVar Summary;
Summary;