I did find a fix for this...zuckerwar had posted this:
1. download the latest version of osCommerce and extract it to your hard drive
2. you'll find the USPS shipping module at includes/modules/shipping/usps.php (dated Jan 9 2008)
3. go to your admin control panel and remove the old USPS shipping module (write down your USPS username and password first!)
4. upload the new usps.php file to your server in the includes/modules/shipping directory and overwrite the old file
5. install the USPS module from your admin panel and fill in the information (username, password, etc.)
You can find the original post here: http://forums.oscommerce.com/topic/327300-usps-module-is-forcing-express-mail-option/