You'll need to create a new project with just the new templates in it, send that to the client, and they will need to import the new templates into their existing project.
For both these steps the 'Import From Project' function will be used. You will need to create a new project (in the correct format) and import the new templates into it, 'File > Import > From Project. Create an xpp of this new project and send it to your client. The client will unpackage the xpp and using the same method 'File > Import > From Project' and import the new templates into their 'master' project. For convenience and simplicity don't forget you can open multiple projects at once using the 'Project Manager'. Right click at the top of the Project Manager tree and select 'Add Existing Project'. I think it may even be possible to copy and paste templates between two projects if they are open simultaneously.
Like some of your other inquires about project and content distribution, the XPression project server would simplify this process as the client would connect to a project server and download just the revisions to the project.
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