I would suggest going yearly, with a monthly subscription there is something more that the client has to think about and they will see bill after bill. Paying up front it is a one time decision.
It might seem that the intial lower cost of being monthly is a good idea, but after 3 months if the customer hasn't setup there site, they will cancel. Paying yearly they will make sure to get the most out of it. It costs the same amount of money to signup someone for a year as it does for a month (in my experience and I have done several similar applications).
every web host and their mother has a tool like this, maybe not as pretty. you have to provide a service that no one else does or does well. You also have to have integration with as many other services as possible. ebay,paypal,amazon and crap like that. wish you good luck.
