archiving in Postbox doesn't remove GMail's inbox label

When I use Postbox to archive a message (which I've already set to move messages to GMail's 'All Mail' folder), it doesn't remove GMail's 'Inbox' tag. That is, when I view my email in my GMail account using a browser interface, the message still shows up in the Inbox. How can I fix this so that I can move messages out of my inbox using Postbox, rather than having to go back to GMail all the time to archive messages?