I am wanting a cell to choose which sum to apply if a value is less than a number in a cell.

For example

D1 value is 12.5

IF D1 >12.5 then I need this calculation =SUM(Postage!$F$5*Prices!E1+Postage!$F$4)
IF D1 <12.5 then I need this calculation =SUM(Postage!$L$11*Prices!E1+Postage!$L$10)

Any help would be much appreciated.

Thank you in advance
Luke