WP Invoices Ultimate is a WordPress plugin by nohalfpixels.
WPIU Has very few options, which include a little bit about you business, notification email subject customisation, and your paypal information.
WPIU uses Paypal, and manual payments. Right now there is no agenda to add support for other gateways, so if you need multiple gateways please look elsewhere.
WPIU uses the Paypal IPN system and can update the “paid” amounts on each invoice when a user pays for it (part payments can be made).
The Paypal transactions are stored with each invoice and can be accessed both in the admin area, and on the invoice page (all paypal data captured).
Invoice Number (randomly generated, can be overidden)
Send Email / Reminder
Invoice Items (title / qty / unit cost / item total)
Page template can be overridden by adding a single-wpiu-invoices.php file to the current theme.