They don't, in this house they're all free.
The next step as soon as I'm confident the technology is reliable is that all calls over (EU) Europe are free with a 2p connect charge.
I suspect the answer is that the cost of the transport of bulk mail is insignificant, like sugar snap peas from Kenya or asparagus from Peru.
The cost of postage is rather like the standard connect charge on the VOIP phone system.
DG