How to inform the user that some data will change value after they hit 'Save' button?

Question

I am designing an accounting page where user can:

1. Select item they bought and add the quantity
2. Then system will auto-populate other fields of the item such as item code and amount
3. User click 'Save Item' and can add another row of item
4. Only when user is done adding all the items, user click 'Checkout'

Issue:

1. The price automated by the system is not the same as advertised.
2. User might be confused as to why the item selected doesn't display the correct amount and did not want to click 'Save Item'
3. But actually the right amount will be displayed after the user click 'Save Item' due to how the back end operates to calculate with the discount and tax (for the mean time, this is fixed)

Question: How can I address to the user that the right amount will only be calculated and displayed after they click 'Save Item'?

Answer 1

This could be solved by simply displaying a feedback message as they start adding rows which will read The final amount will be calculated with discounts and displayed upon saving the task.

It will be a great user experience if you fix the back-end and display real-time values. Until you fix the back-end issue, a feedback message should help the users rely on it and trust the feature.

Answer 2

The user not only might, but certainly will be confused about the different prices. And rightfully so! Expect the users to abort the entire purchase/process and to leave the shop/system/website if this happens. Also expect a large amount of support calls.

If you absolutely need to make it work like that, don't write the "wrong" price in a table column of its own, but include it into the item description. Maybe like that:

Item                                 Amount   Code    HAL

Plunger, pink, 1' 6"                 2        00-47   5
price before tax and discount: $8

Toilet paper, 3 ply                  24       SH-17   3
price before tax and discount: $5

Book "Chocolate cream desserts"      1        YUM-3   7
price before tax and discount: $12

That way it might become clear that the "wrong" price is not a part of the end calculation but rather an additional information which does not enter the final sum. However, this is more of an ugly workaround than a real solution.