Handling on the South Side of the airport i.e. Manx Flyers is not now and has never been mandatory, you just need to make an arrangement to pay your landing fee with finance prior to departure.
This is true, handling is not required (unless using the main apron), however the airport website states the following:
"Outside of the RZ (main apron) there is no compulsory handing regime on the rest of the airfield, but an airfield tenant or club will have to accept the visitor."
So unless you know somebody who is an airfield tenant, you have to use Island Aviation or Manx Flyers. While it is possible to pay your landing fee directly to finance in the main terminal I am led to believe they only accept cheques or cash.