Version 3 of nBill Lite is a major update which introduces mobile friendly invoicing, QR codes, sales graphs, and support for the new EU VAT rules from 1st January 2015. nBill Lite is a cut-down version of nBill (the online billing system), which includes features for creating ad-hoc invoices that can be paid for online. It allows you to create client records for the people you want to bill, or import them from a CSV file or from your Joomla user records. You don't have to create a client record though, nBill Lite includes support for billing someone on a one-off basis even if they are not a registered client.
You can create invoices with an unlimited number of line items, and e-mail them to your client who can then pay the invoice online using a payment gateway (all of the payment gateways and language packs that work with nBill standard edition also work with the Lite edition). If you are based in the UK you can even use the GoCardless gateway for Direct Debits where your client can pre-authorise you to help yourself to payment directly from the customer's bank account whenever you invoice them (no credit card needed).
Please note that nBill Lite version 3 requires PHP 5.4 or above (check the System Information page in Joomla administrator) - if you are still using an out-dated version of PHP, you can use an older version of nBill Lite (2.5.1).