Welcome to GPLDL - we are still beta - please report any bugs via the contact form.
Look like the new API give some issues they updated WooThemes Stamps.com API WooCommerce Extension 2 days ago… Sorry to bother with this!
2016.01.28 – version 1.2.5
* HotFix – Revert back to StampsV45 due to mismatched authentication issues
Have a look at the repository. Latest update is there.