Got around the problem - don't click flat rate button
Site:
I've been trying to print flat rate labels since yesterday, but couldn't get the pricing to show. This worked for me, twice: Instead of clicking the flat rate button, enter in a weight and a value. I used 5 pounds and $1. Then the "get prices" button appears. Click on the Get Prices button, you get the Priority Mail Express choices. So change the pulldown to Priority Mail, select your box size, then go back and click the flat rate button.