Signing documents and sending handwritten notes by fax is one of the very few challenges you will end up facing with an internet fax to email service.  Luckily, it is very easy to work around.

Sign your name on a blank sheet of paper with black ink a few times until you are comfortable with one of your results.  Use a scanner and your favourite image editor (Paint, Adobe Photoshop, or the freeware GIMP) to crop the signature you want to use and save it in a safe place.  When you need to sign a document, load the page into the program and paste your signature overtop of it.  When using Adobe Photoshop, you can make the background transparent by simply choosing 'darken' as the layer style for the signature layer.

Of course, another option is to simply print, sign, and scan the fax back into your computer to send it back.   This might sound complicated, but it is really the same thing you would be doing with a conventional fax machine.