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

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'?