The additional fee is to cover the admin associated with checking your insurance and issuing the appropriate paperwork. The insurance question is germane because, in essence, you ( ie your insurer) will be liable for any damage caused to the aerodrome and its equipment should you have a mishap.
So, can you therefore confirm this 'checking' includes establishing that:
1) the pilot has a valid medical;
2) the pilot has a valid pilot's licence and a valid aircraft rating - certificate of test/check or experience;
3) the aeroplane has a valid certificate of airworthiness together with a valid airworthiness review certificate; and
4) the aeroplane has a valid scheduled maintenance inspection certificate of release to service and has not exceeded the airframe hours or time interval for its next scheduled maintenance inspection?
You can't? On what basis, therefore, do you believe you are establishing that valid insurance is in place?