You’re wise to wait for an official staff response, but Heel-zzzz is correct – you don’t need to buy it for each newsletter you send out. Single use in this case basically refers to a single company, rather than using the same, but modified, template for multiple businesses. In fact, it says so right in the FAQ’s: http://themeforest.net/licenses/faq#email-template-license-a. Here are the only stipulations:
Just like a series (see the single application FAQs), newsletters must be connected, and be released within 1 year of the first installment. There is also a maximum of 52 newsletters released per license.
PS – I’m not a lawyer.
Disclaimer: I’m not a lawyer and I’m not staff (anymore ).
Let’s say you’re building a task management management system and your users pay a monthly fee for access. If you’re using the PSD somewhere inside the paying-customer-only section of your site, you’d need to buy an extended license. If you’re using the PSD only where it’s accessible to everyone, paying or not, you’d only need to purchase a regular license.
I hope that helps. If you’re still confused, I’d suggested reading the examples here: http://themeforest.net/licenses/terms/extended