Ask what sort of resolution and format you'll be getting them in, and be prepared for normal flash drives not to always be big enough. You should also ask about restoration if any are in poor shape, and how the slides will be returned to you (boxes/sleeves/etc).
Generally, most slides are fine at 10-12 MP of resolution, though Ektachrome can resolve more detail to about 40+ MP. And higher resolution TIFF is better than JPEG in case you want to edit the slides afterwards, since once they're digital they can go into a normal digital workflow, and JPEG does clip some even on slide film. If any of the film is negative or panchromatic, you'll 100% want the higher resolution and preferably raw files to adjust afterwards.
Or of course, if you have gobs of time, you can pick up a used Coolscan with a slide magazine and do it all yourself more cheaply and usually with better results. Or pick up that new Nikon slide copying adapter and connect to a copy stand Macro/Micro lens (ES-1/ES-2 adapters, 40-60 mm macro lens), which might be useful to have anyway in case any more turn up and you don't want to go through the process of going to a third party company for a few dozen.