They charge you for the privilege of them processing the customs. They argue that them doing this for you is a "service"
In short they charge you for the privilege of charging you.
Fun fact , if your under the amount to pay customs and the processing fee pushes it over the threshold , they charge you customs on the whole thing.
Once i had something shipped and they did not charge me before the delivered it and then sent an invoice. I filed that invoice in the shredder , that was about 8 years ago now and they have still not chaced it up