It may happen, in the course of your busy life, that you create an invoice but forget to mark some of the items as taxable. Don't worry, this can be done after the invoice is created.
First, load the invoice in the mobile app and tap on the invoice item you want to add taxes to.
Next, (1) tap on the "Taxable" line to make the item taxable, and (2) select all the taxes that apply. Then (3) tap on "Save Item" up at the top.
After you tap "Save Item" it shows you the invoice screen again. Notice that the taxes are now applied to the item. Tap "Save" to save the invoice, and you're done!