I have some doubt about the "net weight" theory, since shippers usually deduct "tare" from containers they provide to clients. (I'm willing to be corrected on that; I am no expert on shipping, and yes, I am not - unlike the Pope, allegedly - infallible.)
I think when a shipper provides containers, it more applies to things like bulk fruit and vegetables, or grain and ore.
I doubt a shipper would have provided the cardboard containers used to pack the Carcanos in Italy. I believe each Carcano was wrapped in heavy paper and placed side-by-side in the container, with some packing material to fill gaps between. Things wouldn't shift much. The overseas shipment would have been gross weight, including tare. In the USA, Adam could have gotten a freight contract for "net weight", arguing the containers were cardboard and that the rifles (TS at first, then Short Rifles) being shipped had different weights.
But if we go with your proposition on shipping weight of the rifles, 750lbs for 100 units, then the rifles would be 7.5lbs each, i.e. 40" rifle, not the 36" model Oswald ordered. No? Yes?
When you say "the 36" model Oswald ordered", you appear to be going solely by the Feb-1963 ad saying the length was 36" (a TS is 36 1/2"). In that case, do you also accept that Klein's was shipping a rifle that weighed 5 1/2 lbs (the TS weighed about 6.4 lbs) and the model-type illustrated (shortened M91 Rifle)?
The Order No and price in the Feb-1963 ad reflect the 40" M91/38 Short Rifle (Adam sold those to Klein's for one dollar more; the TS Order No was C20-T749 for $11.88). Klein's updated the length and weight (albeit wrong but now closer) in later ads the same year. The Order No and price remained the same or near to it. I don't know what year (if any) they updated the illustrated. Possibly Klein's dropped the Carcano mali-order offers in the wake of the assassination, and sold remaining stock on the floor or through an auction.
Oswald ordered the "packaged deal" (Order No C20-750) for $19.95. It makes sense to me that the "package deal" would mean the base model in the same ad to which a scope was added.