Adding a calculated variable to Invoice Document & Checkout layout

I need to display the information “Estimated Import Tax” to the checkout page and the Invoice Document template.

The Estimated Import Tax is a staggered structure (If total is less than $400, tax is 5%. If total is less than $800, tax is 7%, etc).

For the product pages, I could easily change the template to do the calculations from the product price and display it.

But I can’t seem to understand how I can show this information on the checkout page, and the Invoice document (using the HTML editor).

Extra Info
This information is only as information to customers. It isn’t to be used for calculating within the payment section. And so I’m avoiding using the “Tax” function.

For checkout it is required just to edit corresponding template.

As for document, the task is more complex. You can use Gift certificates or Reward points module as example. They adds extra variables for the document editor

Thanks for the suggestion. I’ll have a look at the Gift Certificate and Rewards module to try a understand it. If beyond my understanding, I’d probably have to do it as a customised add-on.