At the moment the user has no possibility to control the calculation process for all amounts of the application. This may cause legal issues as the calculation order may be different for some countries. An example is the tax and discount calculation for items.
The admin should be able to re-order the calculation process in the settings.
The implementation was already started, however some parts of the calculation do not fit and produce wrong results. A forum topic is available for testing including the test results: https://community.invoiceplane.com/t/topic/2667
If you consider working on this please contact Kovah.