I hit a similar problem when I installed extensions for both paypal
and active_shipping. It looks like there is a namespace conflict
between active_shipping and active_merchant. Strange since they are
both written by Shopify. But there is a fork of active_shipping that
fixes the problem. Here is a link to that fork:
http://github.com/djnawara/active_shipping
So clone this into the plugin within the extension. From your project
root, run this:
rm -rf vendor/extensions/active_shipping/vendor/plugins/active_shipping
git clone git://github.com/djnawara/active_shipping.git vendor/
extensions/active_shipping/vendor/plugins/active_shipping
Be sure you have the paths correct for your project!
That fixed the problem for me. Hope it helps you or anyone else who
hits it.
Will Emerson