This would be a killer feature and something I'm perfectly willing to volunteer to help make happen. It's an essential component in my opinion.

I run a web consulting service, so the idea is I do some work, then I create an invoice for that work and send it to the client. They click the link on the invoice, which takes them to my site where they can pay the invoice.

There are all kinds of other integrations we can do if we have that feature. We can integrate with the drupal 'project' module, for example, or various issue trackers.

We can use time tracking modules to record work done and then have invoices automatically generated based on a specific project.

Let's go! We need a solution for this.