In time tracking and billing software you want a software that does at least this: paypal integration, affordable, the ability to upload receipts for expenses, server based time tracking, and customizable templates. There obviously a lot of other features depending on your business, but those should suffice to manage your time tracking and billing.
A few options:
Replicn
Freckle
Or you might want to check out accountsight as it is the most affordable -- http://www.accountsight.com/