For a disc brake kit the only things you need to ship are the spindles, caliper brackets, dustshields, and misc bolts/nuts/hardware. Anyone doing a disc conversion should be buying new calipers, bearings, seals, rotors pads, etc. I just checked Rockauto and Autozone and only $5 core on calipers, so no sense shipping those unless they are really fresh/usable.
So, if wanting to help out a buyer, I would look at shipping the heavy parts in a USPS flat-rate box (~$14 anywhere in US) then ship the lightweight dustshields in a 2nd box that is under $12" dia (12" or larger boxes go by 'dimensional weight' and cost goes up). Athough it might be just as effective to ship all in one box.