Using Membership Plugin/Payment Gateways with Multisite

Hi,
I am currently trying to set up a social networking site using Multisite. I have separate sub-sites for each city with their own Buddypress install, so that users only see members/groups within their respective cities.

I'm having difficulty figuring out how to configure the payment gateways in the membership plugin. I'd like to use the same merchant account to collect payments from all sub-sites, but it keeps prompting me to enter an instant notification URL tied to only that one city's site.

What can I do to make this work?

Thanks!!!!

eg: main site http://www.newgirlcity.com
sub-site: http://www.newgirlcity.com/charleston