OK, I have finally got this thing figured out, and here's what the problem was:

I had moved the entire ZC storefront into a different sub-directory on my site a little while ago. In doing so, I overlooked a couple settings in the configure.php file like the COMPLETE path for example... it was still pointing to the older files. This is why even though I performed the USPS update, that the Toolkit was still finding the old stuff when I searched. I finally noticed that the full path I was looking at in the results was not right, and that tipped me off.

So yeah, I double-checked all my config settings to ensure all references to the path were correctly pointing to the new store directory. Once that was done I went and removed, updated, and re-installed the USPS module one last time and now it appears to be working like it should. Not sure yet if it's rounding properly in every case (more on that soon), but the First Class International option appears to be functioning soundly.

Thanks for taking the time to help, Ajeh, and sorry to have wasted any of your time.