The only fees I have to pay for when redeeming mileage on DL
are the taxes, so maybe it was that that was US$90 and they just called it a processing fee? I find some of MX
's customer service reps oblivious as to what is going on. I've called twice trying to get my SQ
number into their system for my flight in May and both times they say they don't know SQ
and that they are not partnered with them (which, they are go on SQ
's website and you'll see MX
is still a partner with them).
Best of luck trying to get it resolved, for $90 its worth saving the miles and go with an LCC if you can get for that price