I'm curious, has the OP considered the HelloSign API? Is it too expensive for their use case, or is there a {technical, legal} reason that they can't use it?
I really wanted to use it and have emailed back and forth with them several times over the last 2 years (from before they even had a product called hellosign -- when it was integrated into hellofax).
it required account creation for both signers through their site, which forces my users to leave the site to sign, and it wasn't a fit for the UX i wanted.
Similar issue for signnow.com
Benefits of using those though: transfering the responsibility of maintaining legally compliant signatures to a third party. Read more: http://www.hellosign.com/info/legal
For my specific use that's less important, so I opted for a solution I could embed without sending my users somewhere else.
However I think the hellosign folks are awesome guys and i use hellofax for a lot of stuff personally.
