I tried using HelloSign and HelloFax to take care of some contracts while travelling, but the messages sent look like spam (heavily branded, with almost no control over the content of the message), and the document is not sent as an attachment but requires the recipient to register and login to the service.

They're so focused on their viral coefficient that the service is miserable to use.

I dont mean to be so critical, but I exchanged a number of messages about this with their support line, and all I got back was bullshit answers, such as requiring a login gives "better security!" and so forth.

EDIT: The key word I'd use is that the messages are unprofessional. If you're trying to conduct serious business, it's not really suitable, because of their attempts at virality. When it comes to using an API, do you trust these guys to make customer-centric decisions or will you get stuck with something else?