This is a rare casse where I disagree with the other respondents.

An invoice is an entity, and, roughly speaking, a one time event. At the
time of the event it is derived from other data, but, after that, the result
becomes an entity. Like taking a photograph of an event ....at that moment,
it is "derived" from the scene, but then the photograph becomes and entity to
be stored. You can't "re-derive" it later. Your math change is one example
of why not. Another example is if a company changes their address and you
update your db accordingly. Your invoice is a record of where it was
mailed to, which is the old address.

Strictly speaking, DB's such as your current one (and Northwind) do not
store invoices, they merely create and export them as paper or electronic
documents. Under those DB's, if you want to "store" invoices, you need to
store the invoices, which are the electronic or paper documents.

Ditto for transactions and orders.

If the mission of your database is to store invoices within itself, then you
will have to restructure it to do so which will require setting up some new
tables and probably changing your definition of one or two existing tables.

If this route is of interest, you could tell us that and get more help.