See, for example:
https://www.msofficeforums.com/word-...html#post55512
https://www.msofficeforums.com/word-...html#post59393
However, it seems to me you should probably be using a mailmerge to produce your invoices, rather than manually editing anything. Of course, this pre-supposes you have a data source (eg an Access database or Excel workbook) that could be used to produce the mailmerge.