WPDM - File Cart Add-on will help your site users to select random files from different packages and download the selected files as a custom package. This is only will work with WordPress Download Manager Pro. You need to Enable Single File Download and use file_list tag in your page template for button to appear. Note that, File Cart will not work for locked/private packages.
And if you want to keep file cart outside of dashboard , in a publicly accessible page, use short-code