Currently, 25% of my pay is being deducted due to a garnishment. My employer is taking an addition $15 out of each check for the processing of this. He claims it is additional work.

Is this legal?

Each state has its own rules as to what is allowed, I know we are allowed 6 dollars here. You will want to check the garnishmnet paperwork, it should say if and how much is allowed for the paperwork. That does seem a little high but getting a fee for doing the garnishment is allowed.
In all of the garnishments we have had to do on people, the actual garnishment stated what was allowed to be taken.