downtown zone is not part of pier 91. There has never been a 'flat rate' from the pier, however there is one large flat rate taxi firm, which is also the exclusive cab at Seatac.
flatrateforhire.com
or
eastsideforhire.com
both the same company though. Those websites don't work and when I google flat rate taxis a lot of small companies have 'taken over' those sites. Since the 'flat rate towncars' went out of business during covid you basiclly have a ton of guys with one car advertising as 'flat rate' but they aren't really a bargain. Seattle regulates cabs and there has NEVER been an 'official' flat rate from Pier 91. From Pier 66 yes as that is part of the downtown hotel zone. I suggest you book with a larger limo firm or use a shuttle such as
seattleexpress.com
https://a-alimo.com/
is a larger limo firm with reasonable rates which are posted. I'd be a bit careful of using some of the one guy and one car type operations.