The U.S. Postal Service said Tuesday that it intends to purchase at least 66,000 electric delivery vehicles as part of a push to transform its delivery fleet.

The electric vehicles would amount to more than half the 106,000 vehicles it plans to acquire for delivery between now and 2028. The new vehicles will start to replace its aging fleet of 220,000 vehicles, the Postal Service said in a press release.

The Postal Service has faced public pressure from environmental campaigns to electrify its fleet.

In April, environmental groups filed a lawsuit against the USPS for its failure to conduct an adequate environmental analysis before deciding to replace its vehicle fleet with more “fuel-guzzling combustion mail trucks,” according to a press release from the Sierra Club.