... how much does the PCB cost as a percentage of the bill?
A crapload more than a shim, that's for sure.
BUT that isn't the sole reason.
Many meters were sent out with the thinner PCB and the shim solution dealt with the problem with a user serviceable field change. The alternative would be a complete recall - as I don't think Dave or UEI would be sending out replacement PCBs - populated, tested and calibrated!
Sending out the shim with instructions as to how to fit it was simple, inexpensive and only inconvenienced the user for the length of time it took to fit the shim.
The shim solution was found to be good enough to be used in the construction of new units out of the factory - so the engineering has been proven.
Your pedanticism is inappropriate here. The shim solution works well. Please .... move on.
Edit:
Dave: If I've overstepped the mark or got something wrong, please feel free to castigate and correct me!