I have had the same problem. I could order boxes but not labels. I contacted USPS and the replied with the below message. I have not had time to try it yet So I hope it works for you.
Lots of luck,
Thank you for contacting us concerning your inability to order from our site. We recommend you perform the following:
1) Clear your Internet Browser's cache and history and start your session again.
2) If you are using Internet Explorer 6, please lower your browser's security and privacy settings to be at most "medium-low".
3) If you are using any firewall software such as Norton Firewall, BlackIce, etc; please disable the software while ordering, or reconfigure the software to allow sensitive information to be transmitted. By default some firewalls strip cookie information, which in turn limits many shopping carts and their functionality.
4) Lastly, if you have any utility programs that assist in your connectivity such as download accelerators or password retainers, please disable the software. Many of these "helper" programs also run other processes on your machine, which can limit what data is sent to websites.
5) Any sort of novelty software such as special cursors or browser skins may also affect your computer's ecommerce functionality. If you continue to experience trouble ordering, you may want to try removing any software in this category.
If the above steps do not resolve your issue you will need to order supplies by calling Supplies Fulfillment Services at 1-800-527-1950.
We apologize for any inconvenience.
USPS Internet Customer Care Center