This option is for having us take care of installation and basic configuration on your store for you.
Oftentimes, custom themes and extensions tend to override default store templates and functionality other extensions like ours are using in their work which may result in different conflicts and incorrect performance. This is called "compatibility issue".
Including "Installation" option when purchasing a product means we'll install our extension and take care of customizing it to have it work as described under any special conditions created by your custom theme and other 3d party modules on your web store.
If you're not sure on whether or not you'll have any compatibility issues and would like to check how it works first, not a problem, in case of anything you'll be able to purchase Installation package afterwards.