Documentation to be able to add a discount on an order?

[gusman126]

I've been looking for documentation, I could find that you can add a "discount" associated with the customer and the cart, I have not done tests, but I suspect that this discount will be like a coupon or cart discount,

This discount I want to do when choosing a carrier, so adding a discount for each cart or order I do not see it feasible

The modification of the order, making changes by code of total orders and invoices I do not see it either, much code to add a simple discount.

I have looked at other modules of commissions, always positive, in all the occasions I have seen that it was necessary to modify almost all the order by means of code,

I know that you can put other prices and shipping cost in transport, it would be a quick and simple way, but I would like to know if there is something else and that it is simple.