Please keep your download information! You have just paid money for something intangible, why throw away the ability to re-obtain it? If you fear it might be purged from your email, copy the information and paste it into a text file, stored in a special folder.
Second, assume that what you have downloaded is only good for the present. If you keep the installable MSI/EXE, the file you are keeping is likely to be revised at any time, even the next day. While you certainly can keep the executable, do not assume it’s still good a year or two later. Unless the install was downloaded very recently, always go back to your purchase information and download the latest build of it.
In summary, keep your download and always download the latest build of the add-on you purchased before you install it.