I'd be willing to bet there's an issue with the "Use PayPal Profile Based Shipping" option. I downgraded to version 3.3.1 and this corrected the issue.
If you want to install a previous version, go to the plugin page, select the "Developers" link and look at the list under "Other Versions". Select the most recent before 3.5, which is 3.3.1.
Hope this helps!