Web Invoice - Invoicing and billing for WordPress has added supports AlertPay (including IPN) in its lates version.
Web Invoice lets WordPress blog owners send itemized invoices to their
clients. Ideal for web developers, SEO consultants, general
contractors, or anyone with a WordPress blog and clients to bill. The
plugin ties into WP's user management database to keep track of your
clients and their information.
Once an invoice is created from the WP admin section, an email with a
brief description and a unique link is sent to client. Clients follow
the link to your blog's special invoice page, view their invoice, and
pay their bill using AlertPay. The control panel is very user-friendly and intuitive.
You can download Web Invoice and read all about it at http://mohanjith.com/wordpress/web-invoice.html