TL;DR: Use AppScript, refer to
Google's documentation for correct syntax of the SpreadsheetApp class which is what you would use to read a separate spreadsheet.
Use AppScripts custom scripts in the Google Sheets. What you could do is have the script fetch information from another sheet with the info to be sent, then send that info. This would make a ...
See sheet 'INVOICE' in File.
Since cells B:D are merged, there are 2 formulas in B15 and E15 that return Product name and quantity with price for customer specified in cell B10.