With YourSites you can install extensions on to your client sites.
You can install single extensions onto multiple sites at once.
Simply select the target sites on the site list page and click 'Install Extension', select the file and off you go.
Please bear in mind that post installation processing will not take place automatically. So if you install extensions that require a click of a link on the client site or the execution of javascript then YourSites will give you an orange warning and explain what you need to do on the client site directly to complete the installation.