Hi Mike,
First off, many thanks for taking the whole short kit thing seriously.

Still being new to this I can't argue that I have hoards of strip and sheet I can use for spars, fuselage sides and the like. For me it purely comes down to giving as high a proportion of my cash to you and as little as possible to everyone else in the logistics chain.
So short answer.. How ever much of the model you can get in a box where the overall dimensions do not total more than 900mm and the maximum length of the longest side is no more than 600mm. That's around ~35" where the maximum length of any one side is ~23.5". Exceed that and (At least for the UK international postal service) the shipping cost almost trebles.

Not sure how that stacks up for US based international postage rates.
Other than that, what Pete says above. Plan and Ribs are a must have, Canopy and Cowl (If built up fuselage and parts are specific to the airframe). Formers (Again if a built up fuselage). Fuselage doublers. Although electric I'm not adverse to a nice bit of Maple.

, Always useful for LG mountings if not needed for engine bearers. Lower down the pecking order I guess I would put LG, control horns, bellcrank, wheels, spinner. most of those can be purchased locally. Items I would not consider are tank. Most people have their preferred type - and I don't need one, and covering materials, again most have their preferred brand and methods.
You may have opened a can of worms and I would hate to see you spending time you should be devoting to producing kits trying to sort out the best packaging for the (Probably) small proportion of international customers you may have but either way I appreciate the effort.
TTFN
John.