Thanks, Iliyan. There's no other options to select in the plugin so I suspect it's a USPS service issue in not providing separate shipping options within 1st class mail to choose from. I tried explaining the issue to the USPS Web Tools person I contacted but I may as well have been speaking a foreign language since they had no idea what to do.
If I knew MVC well enough, I would code to not allow that option to display. I'll post in the blog as you suggest.
When I configure USPS shipping options, there is only one First Class Mail option to select or deselect but during checkout, two First Class Mail options are offered to the customer - Stamped Letter and Parcel. I don't want Stamped Letter. Is there any way to eliminate this as an option? Thank you!