The starter solutions weren't really intended for "off the shelf" use. They're more of examples of what can be done that you can look at for ideas when creating your own solution. Do a web search for something called "starting point". I believe that the folks that produced the starer solutions for FileMaker Inc. offer a solution where the different files have already been merged.
It is possible to do your own merge, but it requires a good working knowledge of both FileMaker and the starter solutions to do.
Both files have a table of contacts. I would examine the original Contacts starter solution and identify the features I need that are in it that aren't in INvoices. I'd then modify the Customer's table bit by bit to add the needed fields, layouts, scripts, etc. to support the features I want to add to the database. You'll need to analyze the layouts, relationships, scripts and tables to understand how each such feature works in order to replicate it using the Customers table in place of the Contacts table in the Contacts starter solution.
FileMaker Advanced will make the job a lot easier due to its value added design tools.