First of all thank you for purchasing our product. Anyway about the first query donation page is not loading it’s happening because of the page permalink. Please try to reset the permalink in settings->permalink page & it’d work for sure. If not please provide a temporary admin access so that we can have a look at it.
Now about the donation system you can checkout the give plugin documentation about this one. If there’s no option to add surcharge on PayPal then you can add the extra amount in donations bar. Donation can be achieved in a different manner from individual donation settings. If you go to edit page of each donation you’ll have option how you want to get the donations by percentage basis or goal basis. Please check their documentation to learn about give plugin settings.
Hope this would help you build your site. Regards.