Default WPMU Notification email goes to every registered user upon registration...

We have 3 types of registration on our site, paid members (using your Membership plugin), general wordpress registration with the subscriber role, and finally a customer role for people that want to buy stuff (subscribers turn to customers if they buy stuff from our shop).

The problem I'm having is when people go through our shopping cart, and choose the 'create a new account' option (to record their purchase details, etc.), they automatically get sent an email from the WPMU membership plugin. We've customized the message to be specific to one user role, our paid users (members) but everyone gets it.

Is there anyway to select only a certain role that should receive this email?
Or, any other solution that will stop my general (free) subscriber or customer accounts to stop receiving this, without killing it for my members?

Thanks!