Marketpress test before going live

Hello all, my first post in the forum, happy to be here and using marketpress. Great plugin. I really need some help here because I have wordpress installed on a subdomain I setup as a development location for a new website. It is all ready to go and I want to test the shopping cart to make sure it is working. I will change the DNS settings on the client's domain so that it points to this subdomain and then I will get an SSL certificate for their domain name. However, I can't get an SSL for my subdomain and I don't know how to test the shopping cart without that.

Anyone ever run in to this problem and know what to do? I just don't want to put up the new website on their domain and replace what is currently there without knowing that the shopping cart is actually working. Any help appreciated, thanks!