Recently I also had to deal with that "service fee". Basically they wanted 12.50 € + VAT for handling 19.75 € of VAT on an import I made.
I wrote an email explaining that I need to have them provide me with more specific information about what real costs are involved by handling said amount.
The first reply was a generic reply TLDR: "you need to pay because that is what the invoice says"
Then I explained, that usually the cost for a non-interest-bearing monetary bond you would have to supply for a deferment account is about 1 % interest on the deferred amount per year and that I do not see the connection to the amount I was charged.
Got the exact same answer like before.
Send the exact same email as before.
New answer(?): "costs on deferment accounts are opportunity costs"
I sincerely thanked them for supplying me with an irrelevant, but true fact to my initial question and provided an irrelevant but true fact in return, asking to please provide the information I requested.
At that point they just gave up and told me they could not provide that information "for internal security policy related reasons", but would detract the fee out of good will this time. I was set to waste as much of their time and money as possible before paying the requested amount but this outcome is fine for me too.
Sometimes, being the customer from hell can be very soothing.