Where did you set it? It must be in the setup TS-Config of your (shop) root page so it inherits to all pages below. Furthermore, you have to set it in the TS-Config field of the scheduler task as well so it's used in the e-mails.
The tax flag configuration is still not used in your basket page because of "Inkl. ... MwSt". When it's used, "+ ...% MwSt" would be shown instead. Please check in the Typoscript object browser if the taxflag setting is available in your basket page resp. why it's not.