I think it's more like a traditional trial model than freemium - the real benefit of your service is its convenience, not the ability to send letters at all. To benefit from the convenience you need to be sending more than two letters a month, so the free stamps just give people a way to try the service out before they sign up. Anyone wanting access to the real benefit then starts paying (either by default or by switching to a subscription). Nice. Do you have stats on how many people use their first stamp to send a letter to themselves?

On an unrelated note: why isn't there a nice big picture of your gorgeous-looking paper and envelopes on the site?