How to tell Stripe to send Invoice?

I am using Spark and want to have Stripe send the customer an invoice instead of using Spark's built-in emails that don't very good.

In this documentation is says you need to send the customers email to Stripe. https://stripe.com/docs/receipts

What is the best way to do that?

I tried overriding stripeOptions() in the User model to include the user's email but that did not seem to work.