When did the U.S. Postal Service completely transition from horse-drawn to motorized transport?

Question

Simple question, hopefully not too broad. When did the USPS stop using horses & horse-drawn carriages for most of its transport and delivery? Were there any private competitors (such as modern-day UPS and FedEx) in operation at the time that this transition was taking place, and if so, who was leading and who was following in the transition?

Answer 1

The USPS actually still uses horses for some of its routes, particularly those in very rural places, like the bottom of the Grand Canyon. https://www.cbsnews.com/news/special-delivery-mail-by-mule/ However, after instituting Rural Free Delivery in 1890 and implementing in nationwide in 1902, horses were phased out in the 1910-1920 time period, with the USPS operating the largest civilian automobile fleet in the world. https://history.stackexchange.com/questions/27609/when-did-the-u-s-postal-service-completely-transition-from-horse-drawn-to-motor