I've read this topic with interest. It's been my experience that there is not a single mechanical device on a car, including carburetors, that a motivated amateur cannot service in a completely satisfactory way. I'll be the first to admit that I've not opened up a Zenith carburetor since I had a Model A Ford with one in 1960. But all modern carburetors are complicated and require that all the adjustments be made in the sequence specified in the instructions that come with the kit, or they will not work properly. How complicated can it be to follow a set of simple, well illustrated instructions?
As for bushing the throttle shaft bores, that's hardly rocket science. The shafts and throttle plates are removed, a straight rod placed through the shaft bores and chucked in a drill press or milling machine, the carburetor body is then clamped in a vise secured to the table to assure bore concentricity with the machine spindle, a sharp, straight drill is used to increase the hole size to the bushing OD, the bushings are pressed or tapped in and the shafts and throttle plates are reinstalled.
It's amazing to me that anyone would pay $500 to have 2 carburetors disassembled, soaked in solvent, the passages blown out with compressed air, and throttle shaft bushings and kits installed. Wow, that's really easy money!
Dan's Zenith invention sounds really interesting even though the carburetors on my vehicles are Weber, Carter Thermoquad, Carter W-1, Rochester Quadrajet and Edelbrock AFB clone manufactured by Weber. Except for the Webers on my wife's 190SL these carburetors have all had their jetting optimized by seat of the pants tuning and plug reading and work very well.
Here's a link to an interesting article on plug reading. There are others like it too.
Spark plug reading
While I would not take exception to your comments, they make the assumption that the OP has access to the required tools, equipment AND experience to perform these operations, which they may not.
Also, while something as simple (to you) as rebushing a throttle shaft might be an hour in the garage on a Saturday afternoon, for someone who has never done this the level of confidence is very low, not to mention the potential for a very expensive and potentially catastrophic failure being quite high.
As a former NIASE certified mechanic who made a living working on high line cars, I recognize the value that a competent and professional repair has for an owner who might be reluctant to take on what you or I might deem a "simple job."
I have rebuilt many Zenith carbs, and while $250 to do so might seem extravagant, I can easily justify the expense based on accumulated knowledge and the cost of the equipment to test and adjust the carb.
Having the carbs bench and flow tested is a big plus - that's not something one can do at home, and it eliminates a lot of possible concerns during the setup and adjustment.
Based on what i know about "retail" carb rebuilds/repairs, I think $250 is reasonable considering the value the OP is getting for the money. Not only that, the responsibility for proper operation and repairs now falls at the feet of the rebuilders. If it was you or I that is our time trying to figure things out. In this case the OP has transferred that responsibility to the rebuilder for a fee.
In this day and age I'm surprised one could even find someone to rebuild these carbs. I say "Go for it!" and get the car running well.