It's unusual to not see an aC staff respond on a thread like this. Either they've missed it, or they're working on a solution that will blow our minds.
It would be great to have the ability to invoice directly via activeCollab. I like the idea of an add-on module (AppsMagnet?) or integration to a 3rd party service. Though the monthly fee/micropayment model of Freshbooks rubs me the wrong way.
Would much rather pay for it upfront the same way we pay for aC.
Invoicing is already part of activeCollab in form of Invoicing module. You can read more about it in User's and Administrator's documentation in Docs section of the website.
I think I was reading more into this thread, and maybe I haven't fully explored the invoicing module yet.
With all the comments about Freshbooks, I thought that people were looking for a click-to-pay service for aC. It would be great to have the ability to share a link to allow payment via PayPal or other merchant service.