The 'Tax' line items calculate against all taxable line items ABOVE them. So in order to have the current tax line items update, insert the new row of charges ABOVE them. Or you can add more tax line items below the new charges.
The charge grid is very editable. You can insert/remove rows anywhere you want. The 'Taxable' flag only signals tax lines below to calculate on those rows. The tax lines can be anywhere on the grid.
The guest does NOT receive a new email automatically for the new charge. We don't do that because we don't know if that's your intention. Often, the owner only wants to adjust the charges (fixing typos, moving line items around) without alerting the guest.
You can certainly send them a manual email on your own, explaining the new charges.
You can also collect a payment by clicking on the Payments tab (under their booking) and clicking the "Collect New Payment" button. Any credit card they've used in the past will be on file for a quick collection.