Are you setting for your market is excluding tax? And why do you want to show the SavedAmount in type of including tax if your market is excluding tax?
In Commerce, all amount are calculate base on the setting of price is including/excluding tax in market. It mean that if your market is setting the price is excluding tax then the SaveAmount of shipping discount is excluding tax.
@Giang - yes, prices in the market are excluding tax. However, I interpret this simply as, prices are persisted as net prices (without tax), which means that tax calculators will add sales tax upon the net price (instead of deduct tax, if the prices were including tax). The setting is named "Prices include tax", not "Show prices including tax", which I think is a big difference in this case.
So - to answer your question "And why do you want to show the SavedAmount in type of including tax if your market is excluding tax?";
We want to be able to import net prices and let Episerver apply sales tax upon the net price - which I think is reason for the setting "Prices include tax". We still want to be able to show prices including tax (and discounts) and that those numbers (which match what the customer paid) are persisted even after an order has been placed. I don't think this is a unique setup.
For your case that "let Episerver apply sales tax upon the net price", you need to set to prices excluding tax. If you want to show prices including tax (and discounts) you should handle it by yourself because as previous my comment, EpiServer will calculate all amount (price, discount, shipping cost, ...) base on the setting of price is including or excluding tax in market, this setting could not be combined.
Alternatively set the market to have prices including tax and write your own import that applies tax to net prices.
In my opinion prices excluding tax is really built to be used in business to business, B2B, sites.
Well, my client sell to both B2C and B2B, and to have prices including tax, in for example US, would mean that product net price would be different from state to state. Would be nice to have one setting called Prices include tax and another setting called Show prices including tax. Having just one setting is not detailed enough.
Then go with Giang Tran's suggestion. For having more than one setting - post in the "Feature Requests" forum and hope for the best. Even if it happens it will probably take time you don't have so don't count on it. :)
I have created the promotion Spend amount to get free shipping. In the checkout process I want to show the discount that the customer will receive, but if I look for applied promotions on the cart, the SavedAmount property is specified excluding tax. How can I find the corresponding value including tax, and can that solution work for both carts and purchase orders?
The shipping tax rate could theoretically change over time.