Our built-in clipping service works across all many OS X apps, but the Postbox folks took the additional step of adding their own "Send to OmniFocus" feature.
Here's a link to
their announcement from when they added the feature; I used Postbox some while they were working on this, but I haven't used it much since then. If you contact their support folks, they can probably help if you have any trouble with it. Hope that helps!