For payment terms and condition, go to
Sales Invoice Template, then type text in the box. This text will appear as a footer on all invoices until they are paid in full.
For a comment such as your computer-generated invoice example, enter the desired text in the Notes field as you create a sales invoice. To have the same note appear on all invoices, click on the blue “set default” link and fill in the desired default text. All subsequent invoices will begin with that text in the Notes field. But you can overwrite or delete the default text as you create an invoice, so this feature is useful if you frequently, but not always, include the same note.
For your authorized signature, you can type in the desired words and create an underline, as well.
For more exotic changes, see the Guides concerning
View Templates. Changes made via that route require HTML knowledge.