How to add a order discount automatically in checkout?

Hi guys, 

In the website which I am working with, there is a scenario. A customer may have a special payment term e.g. always has 10% off in every order he placed.

I am considering creating a promotion 10% discount and apply the promotion in checkout. The problem is there are various discount percentage on different customer.

Any suggestion how to implement this feature?

Thanks, 

ChiChing