Customer and Invoicing plugin for Wordpress site

I need a plugin for a wordpress site that will allow a car dealer to create customer records and send invoices. I have tried to customize the WP-Client plugin, which has the right features, but we need to add new fields to the invoice, and it doesn't seem to allow that in WP-client. We just need a very simple plugin. The customer record has about 15 fields - all just simple text boxes. The invoice also has about 15 fields - just text boxes. and then we need to export the invoice as a PDF. We don't need any payment gateway. The invoice will just be printed and sent to the customer. The customer record and invoice needs to be integrated, so we can pull the customer details into the invoice.i.e. when creating an invoice we select the customer name and it will have his details. Either we can try to customize the WP-Client plugin, or build a new plugin from scratch. WP-Client has more features than we need. What we need is very lightweight and simple.