Discount on Total Bill Amount

Hi,

I am not sure if this question is answered previously.

I want to implement billing discount mean to give discount on billing time.

Here is the examples.

Total Bill = 200 USD (which include usage + MRC + tax + etc)

If total Bill > 200 and Bill < 500 give 10% discount i.e 20 USD, so the final bill after discount is like 200 - 20 = 180

If total Billing > 500 give 20% discount and so on.

Any suggestion?

Thanks.