Could you add a document about adding an extra payment fee as a new total ?

### Is there an existing issue for this?

- [X] I have searched the existing issues

### What's missing?

In the Customize Checkout guide https://developer.adobe.com/commerce/php/tutorials/frontend/custom-checkout/, it would be great if you could add a tutorial on how to add an extra fee properly.

On the web you can find only very basic examples of how to add the fee, but no details on how to properly present it in the checkout page or how to apply correct tax calculation. Many parts will work but the experience in the checkout page is not consistent. Other areas show the fee, other areas don't show it.

It would be great addition if you could showcase an example for creating a custom fee that adds a flat amount for the whole order

Start with the existing Customize Checkout guide at https://developer.adobe.com/commerce/php/tutorials/frontend/custom-checkout/ and review how checkout totals and tax calculation are documented. Done means adding a tutorial that demonstrates a flat custom fee for the whole order, explains its tax treatment, and shows it consistently throughout the checkout page.

Written by the indexing model from the issue text.