Shipping can be so cheap as they collect so many articles until they have a very big sea-container full of stuff. In such a container fit ~80.000 items .
biggest = 13.6 x 2.7 x 2.3 meter ~~ 85 m3 == 85.000.000 cm3
A typical box for electronics is 5x10x20 cm = 1000cm3
so there fit 85.000 packets in a container.
If you pay 10 cents per item (less than 5% of the average price) for transport that means you have $8000+ for transport of one container.
How many containers on a big ship ==> 1000?
So that makes in the order of $10M for one tour (OK it uses lots of petrol too)
This is a first order estimate of free shipping (now you may shoot 