Btw. here's a link: http://www.duetapp.com/

Personally I wouldn't ever use a SaaS for my core business! With Duetapp or other opensource project for example, I am able to modify the source it. (opensource!=free)

Regarding duetapp, I think commerce and organization should be seperated. But no other invoicing app does that atm. anyway. However I feel that this is the last bit required to make it almost perfect. Basically I would create a clean and modular and payment gateway, so that I can be used by any other app too. Write a little duetapp plugin and voila. Much better than integrating every single payment method manually into system A, system B etc.

Referencing my comment below. Here's a phnatomjs example that will render a webpage to pdf:

https://github.com/ariya/phantomjs/blob/master/examples/rast...

Note how that doesn't create a PDF.

Note that pretty much everyone has a PDF printer driver installed nowadays.

How does one go about creating (and sending) a PDF from a web form (like Invoice-o-matic and billable.me)?

I would take POST data, and plug it into a templating system (like Django's), which is using something like LaTeX to define the basic properties of the document and populating with data and logic from Django.

Basically a mix of two markups in one file. Meh, there's probably a better way to do that.

Eh, easy. http://jspdf.com/

You can create a file with javascript at the client side, without creating load on the server.

phantomjs could fix that for you.

http://phantomjs.org/