Hi, I'm new to Odoo, and have been evaluating the POS module.  What we'd like to have is a button on the module that would apply a 3% fee to the total order or a user-entered discount to the total order.  I see discounts per item, but not per order.  And I don't see a way to add a percentage fee (the opposite of a discount) to the total order.  Is there a way we can add a button somewhere to do this in the POS module?

If it requires custom code, that's something I'm up for doing, but I'd love a pointer to a tutorial on customizing, specifically for adding a button to the POS module (I couldn't find anything online so far).

Thank you.