Can anyone tell me why I can ship a 50 pound box for about 35% cheaper than I can ship 2 - 25 pound boxes that are half the size? Its the same total weight and volume, same destination, just different prices.
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Allow an example:
JLS shipping company charges $2 per package + 10cents per lb.
2x25lb package would be $9 (2 packages thst each cost 2 flat fee plus 2.50 for weight).
1x 50lb package would be $7.
For this reason, you generally want to try to pack as much as possible into each package as allowed by Amazon's standards.
Trucks, planes, and boats are all constrained by cargo volume as well as by weight. If you have too much weight, or too much volume out of proportion, the container isn't as full as it could be. You will find the two boxes take up more room than one big one. (multiply the length times width times height in inches to get the volume in cubic inches)
So shippers know lightweight, low density boxes take up too much room in the shipping containers to be as profitable as smaller, denser parcels. To compensate, shippers have gone to a system were a standard density box is defined as 166 cubic inches/lb. Anything less dense is billed at the higher weight a "standard" density box would have weighed at that size.
It makes sense if you think about it, it is to your benefit to make your shipments as compact as possible, so that they are a better fit in cargo.
In a much less straightforward manner, you can also save future lightweight penalties if you add your lightweight stuff to an especially dense shipment, to fill the 166 cubic inches you are given per pound. The same way a truck close to max weight but not full volume wise could add some lightweight but bulky things without adding much more to the overall weight.