Boulder, Kansas, in Arapahoe County was granted a post office in April 1859.
Unlike close by Auraria, Kansas, though, Boulder was not in Kansas, it was in Nebraska.
Auraria became Denver City (named after a Kansas governor) and is now a neighborhood district in downtown Denver.
For some reason, the post office department chose to include Boulder with Kansas Territory.
Both Boulder and Denver City were in Colorado Territory by the beginning months of 1861.