Just don't let the shop put actual brake fluid in the system when flushing . . . that will indeed make a major issue.
Shop needs to keep fluid in the reservoir, there have been some incidents of debris in the tank getting sucked into the pump, so keep it with some fluid level.
It's almost certainly the hardline that
@NYC570 is referring to, and there are a couple threads on here with folks working on that / some with shops and some DIY.
There are more of these coming as rust gets to this location over time, so please add on what parts you did need to replace, and what the labor was so we can baseline and build on this repair to help others.
The total costs should be labor + hardline (whichever one it is) + fluid to flush and find the leak with. Maybe some incidentals.
And as
@turbo8 said, if you want to pull the system it's not a big deal for an experienced suspension shop and the basic parts aren't so much. Of course an upgraded suspension that can rival the comfort of AHC will be more than basic, but you could plan for that over time. And no fixed suspension is dynamic, but not every owner uses the height control much.
Also consider a rental, when talking thousands of $ on each decision a few hundred in rental to relieve the decision pressure might be a good investment.