Compare with labor costs for either factories or small owner operated export businesses run out of Shenzhen and you can see where there might be a pretty big difference in the handling price.
For these small shipments, you would get a shipping list, see that the order system determined that it would fit in an envelope, grab an envelope, gather the components (perhaps for ten envelope-orders at a time), place in envelope, slap a label on, and place it in the outbox for envelopes.
It doesn't seem that much more labour intensive or more expensive in terms of packaging, and the first distributor in a geographical area to do so might gain a bunch of customers. Even if the base price of components was increased by 10% or 20% for envelope orders a lot of us would still use that service.
I wanted some discrete through-hole MOSFETs for bidirectional level shifting as described in this application note [1].
The ZVNL110A satisfies all of the requirements [2]. In small quantity they were $0.604 each at the time from Mouser, and I wanted 10, so $6.04 for the MOSFETs. And then another $7.99 for freaking shipping. (I ended up ordering 30, figuring maybe I'll eventually need that many, and then I'm "only" paying 50% for shipping instead of >100%).
These things are very small, and only weigh half a gram each. Just take an ordinary letter envelope, drop them in, put a first class postage stamp on it ($0.49), and mail it. A $0.49 first class stamp allows up to 99 grams, so orders up to a couple hundred of these could be shipped that way.
For those curious, the size constraints on a first class letter in the US are:
0.25" is 6.35mm. A large number of through-hole components would usually fit, such as transistors, diodes, resistors, and disc capacitors. Most surface mount components would fit.[1] https://cdn-shop.adafruit.com/datasheets/an97055.pdf
[2] http://www.mouser.com/ds/2/115/ZVNL110A-95598.pdf