Suppose I sell web hosting.

Every time someone subscribes to a plan, I want to add the web site domain so it can be automatically configured when I get the subscription_created event from Zoho subscription.  

If that person takes 5 extra emails as add-ons, where could I store those emails to be created automatically?

We can add custom fields to customers, and invoices, but not to subscriptions.

What's the best (robust) way around this?