Besides being a well-established startup in the digital signature space and writing all their software in Haskell, Scrive has a good API. I have used it myself for a small project, to build a petition service, and it was very straight-forward. Of course, their typical API user is a large organization with a lot more complex requirements.

Fancy marketing presentation: https://scrive.com/gb/en/scriveapi

Documentation for devs: http://api.scrive.com