I have just discovered thet according to accounting law here, invoices no longer need to be stamped and signed,
Yes.
It is sufficient just to send them by email.
I email the ones I get but my accountants want the originals.
But I email my invoices to other people and they print it, no problem.
E.U. law is changing Polish law and a lot of accountants are not aware of the changes and unwilling to change.
The accountant is probably just covering his back, if the tax office go in to check, the accountant probably thinks it would be easier for him to have the originals. Although in reality the tax office cross reference for confirmation.
Polish bureaucrats love their stamps :)