Someone is confused...
We were just on the Summit 3 weeks ago...and had early seating...no problem, no "fees"...
My first thought was that, if this were on a Solstice Class ship, then we might be talking about upgrading a cabin to Aqua Class--which would switch Dining from the MDR to Blu...and that, perhaps, Blu had early still available...
But, this is the Millie...no Blu...
You cannot "buy" your way into early seating...either they have the space or they don't...If they don't, they put you on a waiting list...You can get some priority if you're Elite Captains Club...but, even with that, there needs to be space...
Of course, I've never seen any seating be entirely full...Sign up for whatever...when they get to the ship, they should go see the Maitre d' right away...
Trust me, some folks find themselves in Early and wanted Late...or they realized they'd rather have the new open option...in which case, space opens up and they will accommodate you if they can ...and chances are they can...
But, I've never heard of a "charge" for any seating time in the MDR...