Hi, 

In Odoo, we can add discounts and promotions to the sale order.

To know more about discounts, refer to the following.

* https://www.cybrosys.com/blog/how-to-manage-customer-rewards-discount-and-loyalty-in-odoo-18-sales

* https://www.cybrosys.com/blog/how-to-apply-discounts-and-margin-analysis-in-odoo-18-sales

* https://www.youtube.com/watch?v=Zmxc_bRj_kM

Hope it helps.

In Odoo, applying a discount or surcharge to the whole order isn't available by default in some versions, especially in the Community edition. But no worries — you have a couple of options:

1. Custom Module: You can develop a small custom module to add a global discount or surcharge field at the order level. This field can then be used to adjust the total amount before confirming the order. It's quite flexible and can be tailored to your needs (e.g., percentage or fixed amount).
2. Buy a Module: There are several third-party modules available on the Odoo App Store like "Global Discount on Sales Order" or "Global Surcharge". These modules are ready to use and might save development time.

Let me know if you want some guidance on building the custom module — happy to help!