USPS Express Mail Won't Show

I have enabled USPS and setup a USPS Express Mail shipping method, but it won't show up in checkout. I found a post that mentioned their username was wrong, but mine is ok. And I've contacted USPS to have the username go into production. So any idea why this method won't show up at checkout?!

Just discovered this is something specific to the Express Mail option because priority mail works just fine. One thing is that the only option for Express mail is a letter … but Express mail isn't just for letters.