IF condition

I am creating a product order form.
I have a field with a formula which totals all the quantities, multiplies them by the unit price and gives a total price.
I also would like to have a condition that IF the total price is less than a certain amount then a fixed delivery charge will be added automatically.

I don't think I can add this condition to the formula in existing 'Total Price' field. So I created another field and attempted various permutations of possible code for the IF condition. But none of them have worked.
I cannot find an example elsewhere which equates to my requirement that I could copy.
Thanks in advance if there is anyone who can help!!