How to Register a User but Not Take Payment in MarketPress


Could you please suggest a good method for having prospective customers click the Order button on MarketPress, but not complete the order? I am trying to see if people will buy this product before I develop it (existing market) by purchasing Google Adwords to test the market. I would like to capture their email information however, so I can follow up with them whenever I complete development of the product. I attempted to do this by adding in arbitrary PayPal api, and it looked like it was going to capture the email details, but it did not save them because the order was completed.

Thanks in advance,