How do I add notes that always show on my invoices or estimates?

Use the default notes field. Go to Business settings → Invoices (and Estimates, separately) and fill in the Notes field. Whatever you put there is pre-filled into the notes section of every new invoice or estimate you create — payment terms, thanks, legal disclaimer, anything you want at the bottom of the document.

The notes are a default, not a lock — you can still edit or remove them on any individual document before sending. Changing the default later only affects new documents; existing ones keep whatever was on them at the time.

If you also want bank account or payment instructions in there, see Adding bank account / payment instructions to invoices. For the full set of invoice customisation options, see Customising invoices and estimates.