HI !
I would like to propose to you an alternative solution, one that would not require you to develop a VBA macro script. It would involve an additional Sheet to list down all possible values, without duplicates, for Product Name, Rate Plan, and Sales Person, and/or Invoiced By or another parameters that you might wish in the future.
I intend to use the "Data Validation" functionality of MS Excel, setting up individual validations for each of your required parameters. The sum shall be done using a relatively simple SUMIFS formulation that can be edited by the end-user, as required.
Please drop me a line, if you are interested. I hope to discuss further your requirements. Thank you in advance !
Best regards,
Tristan