Hi Playman.
Making sure the correct sales tax is charged on your invoices is imperative to keep your accounts correct and able to file the right information with the CRA. I'd be happy to help you with getting your sales tax entered on an invoice.
When selecting a sales tax if you chose excluded that means that the sales tax is not added and would show up as 0. In order to have your sales tax show a value you'd need to select the correct sales tax code for the province. I recommend following the steps in this article on how to set up and use sales tax.
If you have any questions let us know and we'd be happy to help.