From 5W Infographics, a breakdown of how the bargain basement carriers do it:

AUTHOR
Mick Weinstein
Mick is the Head of Editorial for Covestor, a financial journalist and online content specialist. Prior to joining Covestor, Mick was for five years the Editor in Chief and VP Content at stock market analysis website Seeking Alpha, where he built the editorial function as the site attracted over 3.5 million unique monthly visitors and developed an innovative platform for intelligent stock market discussion. Mick is a graduate of the University of Michigan, Ann Arbor.
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