Everyday US Citizens are bombarded by bulk mailings sent by companies to solicit goods to consumers. This mail is called "junk mail" as most is discarded as trash. What small proportion is not recycled, only ends up in landfills.
Additionally, the US Postal Service provides this "service" at an extremely low rate, which encourages the further use of "junk mail" as an advertisement source. This method of advertising typically has a very low return rate. Given the very low return rate, the cost in environmental factors such as increased landfill space, fossil fuel consumption for delivery, and finally the impact of vehicle emissions during delivery, do not justify the return in revenue for corporations.