Zuora, one of the startups that provides on-demand software to manage companies’ billing and payments, aims to take advantage of growing interest in cloud computing with a new product dubbed Z-Commerce, which will provide Zuora’s services to developers on cloud platforms like Amazon Web Services and Google App Engine.

The Redwood City, Calif. startup has already released three products: Z-Billing, which automates the billing process; Z-Payments, which integrates with PayPal to handle the actual payments; and Z-Force (perhaps the only Zuora application whose name isn’t completely self-explanatory), a billing and payment system that integrates with Salesforce.com. What Z-Commerce adds to the mix is as much about audience as it is about functionality — where the other applications targeted a company’s finance department, Z-Commerce is meant for developers to use directly.

The new product includes application programming interfaces (APIs) that let cloud developers access Z-Billing, Z-Payments, and Z-Force; the ability to add one-click order processing to any web site; a full web store; sample code; and more.

Like many other tech commentators, chief executive Tien Tzuo says this is the year cloud computing will really take off. The platforms are here, or will be soon — Amazon, Google, Salesforce’s Force.com, Microsoft’s Windows Azure — but what’s missing is a way for most of the developers to make money. That’s where Zuora’s “business cloud” comes in: Developers can focus on their product while Zuora handles the business infrastructure.

This is a direction that makes sense: As venture capitalist M.R. Rangaswami noted last year, one of the big opportunities for startups lies around services surrounding the big cloud platforms. Interest in billing systems delivered via the software-as-a-service business model seems to be picking up, too. Aria Systems just raised $8 million, and Vindicia is another competitor.

Zuora has raised a total of $21.5 million from backers including Marc Benioff, chief executive of Salesforce.