Having a very similar issue to this one:
I'm using marketpress for payment gateways, we want a manual payment option that automatically confirms (b/c the person will be paying upon arrival).
I tried the membership suggestion listed in the thread I linked to, but despite checking to allow a certain membership level to not have to put a deposit down in my appointments settings, it still throws the appointment into "pending."
Another membership related question that I was failing at figuring out. Can you automatically put anyone who registers into a certain access level? The second question is moot if the first question doesn't actually work as a solution.