From NCH Software:
Express Invoice Free for Mac OS X business invoice software. Billing software made easy. Includes remote web console access so multiple users can generate invoices, manage clients and generate billing reports. Express Invoice for Mac allows you to create recurring invoice templates for ongoing services and link to credit card gateways to easily process payments for goods and services. Express Invoice for Mac also includes a number of standard reports such as unpaid accounts, payments, sales person, accounting reports and more. You can then print, email or fax your invoices to your clients directly.