Do you think it's best for YOU to send the email directly to my customer, or really shouldn't it come from me? What about the confidentiality of my customer records. You mention 3GB of storage, are you keeping these invoices around on your server?
The Stripe API is powerful, but you definitely open up your data to 3rd parties when you leverage it like this.
Could you bundle a 'self-hosted' version which could be run internally?