The eBay unit rolled out features on its site earlier this year, enabling users to create and e-mail invoices, accept card payments, and track and manage these invoices and payments.

In a blog on the PayPal developer network, Srilakshmi Mudigere says the feature has proved popular but merchants, developers and business service providers have asked for the ability to integrate directly with existing applications such as order processing, inventory management, time tracking and CRM.

In response the firm has developed APIs for creating and sending invoices, authentication, encryption and integration with PayPal Permissions Service for third party authentication. APIs to update, search, retrieve and cancel invoices and to record offline payments are on their way.

Separately, the US Treasury has signalled plans to switch all supplier payments to e-invoicing by the end of fiscal 2012, in a move the agency says will halve processing costs by $7 million annually.

If adopted government-wide, it could eventually save up to $450 million annually through lower staffing costs, according to official estimates.