If you're prepared to post the required number of posts on the forum your Post2Host provider asks you to, as part of your hosting agreement, then you should be fine.
If they offer you the amount of space you need, and an acceptable amount of bandwidth, etc. for your needs. And all you have to do is post, say, 50 topics a month on the forum, then as long as you do that you should have no problems.
In my opinion though, having to post 50 topics (or posts) a month on a forum would get a bit tedious. I'd prefer to pay and just get done with it. No time to sit and talk.