You can do this by adding a formula in your Document Title column and pull the <invoice number> and <customer name> from specific columns.
In the case below the formula (1) will be:
="Invoice "&C2& " for "&D2
Copy and paste the formula on the rows where you wish to generate documents.
Map the Document Title to this header column in your spreadsheet (2)
NB: Only copy formulas to cells where you wish to generate a doc and/or mail merge as the add-on will run on every cell where data is detected.