From whatsnew_1.5.0.html file in the zencart 1.5 docs folder:

USPS module removed from core in favor of being an addon, due to the volatility of frequent changes made by USPS. The addon is available in the Free Addons section of the Zen Cart website.

and from changed_files-v1-5-0.html:

DELETED Files (since release of v1.3.9)
/includes/languages/english/modules/shipping/usps.php