Hello,
I had some issues trying to get the USPS shipping modules to work that came with OS Commerce version 2.3.3 -- it only displayed "Library Mail" which I'll never use.
So, I made several copies of two files:
catalog/includes/modules/shipping/usps.php
and
catalog/includes/languages/english/modules/shipping/usps.php
Then modified them to display only USPS First-Class Parcel, USPS Priority Mail, and USPS Express Mail. They're new files, so you can simply drag and drop them into the appropriate folders without destroying your website. Once you've uploaded them, you can select each USPS shipping method in the Admin panel (Modules -> Shipping) then "Install" each new shipping module.
You can also alter the text that appears in each shipping method during checkout. Just go to you Admin panel (Tools -> Define Langauges). Then scroll down until you see "modules/shipping/uspsParcel.php", "modules/shipping/uspsPriority.php", and "modules/shipping/uspsExpress.php". Click on each one to change/redefine the MODULE_SHIPPING_USPSEXPRESS_TEXT_DURING_CHECKOUT text.
Each USPS shipping method appears as its own shipping entity during checkout:
I hope this helps other people who've been running into the same issues I did.
Feel free to update and re-post better/modified code.
Thanks,
Matt