Install your Paid product.

Simply right click on the attached file and select ‘save as’ that get’s you a menu and you can save the file.
Once the file has been saved, open the folder were the file was saved and double click that file.
That will register your paid Avast product.
You can follow a video. It may not be for your product but it works the same way:
https://www.youtube.com/watch?v=3BVdFGoFo0s