Cloud billing provider Aria Systems announced today that it has raised $20 million in its third round of funding led by InterWest partners to double its staff and ramp up promotions for its online billing service.

Aria Systems specializes in providing companies with the tools to create a billing system that, for the most part, customers will never see. It gives companies a number of ways to implement billing systems for their services, like monthly subscriptions or one-time premium payments.

The billing fees are either charged as a monthly fee or are based on the number of invoices its clients issue in the billing service. Aria can also take a small portion of revenue generated by each client through its billing systems as payment. About a year and a half ago, more than half of the revenue the company generated was through a revenue-sharing model, said Aria Systems’ chief executive Mike Morini.

Today, most of the revenue Aria generates is through charging its clients based on the number of invoices filed, Morini said. That’s because non-traditional models for billing — such as the freemium model, which gives users a taste or a stripped-down version of a service and then charges for premium add-ons — have risen up and taken over the space that subscriptions and premium purchases used to dominate.

Aria’s technology is primarily deployed with larger enterprise customers. It provides smaller businesses with billing tools through its partnership with PayPal. The next step for Aria is to package the offering as a white-label service — meaning they can sell the software service to another company that will then re-brand it as their own. Aria has to go through a number of rigorous steps to make sure its information is secure, so there aren’t a lot of concerns there.

The San Mateo, Calif.-based company has around 50 employees but plans to expand to 100 with the new batch of funding. Existing investors Hummer-Winblad and Venrock also participated in the round. That brings Aria’s total funding up to $34 million after two earlier rounds of funding.