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)
Your generated documents will be titled according to data in the Document Title column
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.