You can add notes to your invoices, quotes and purchase orders by using the Notes field at the bottom of each document. The text in this field can be formated as you wish (bold, italics, bullet points, etc.).
Please note that what you write in this field will only appear in this document in particular and not in every document you create with LSP.expert.
To automatically add the same note in all your documents, you should use the Terms field. See this article: http://help.lsp.expert/documents/where-can-i-add-my-terms-conditions.