That's pretty much how it happens in Romania. All (or almost) salaries are negotiated post tax (so what gets payed out to me) and the employer is responsible for paying the appropriate taxes to the government.
In that case you have to go to the tax office and file out some documents and then you pay the taxes for the investments separately.
There is a single income tax rate, 10%, so it doesn't matter how much you earn. If your investments make more than the annual minimum wage, you also have to pay the health insurance tax and "social security" tax.
