Pricing is based on usage, defined by monthly email volume. Your payment changes month to month based on how many emails you sent that billing period. The rate is $1.25/thousand emails sent for a movement partnership (minimum $15/month) and $2.50/thousand emails sent for a network partnership (minimum ($125/month). You can get a quote for your email volume here.
We use a subscription based billing tool, Chargebee, to bill our partners based on monthly usage. Whichever email is used to set up the initial payment will receive monthly email receipts, though you have the ability to add additional recipients. The payment amount will change month to month but will be processed the same time each month to ensure your group has access to the partner features.