Ask HN: Good Employee Onboarding Tax Form Automation Software Options?

Our business is looking for a good solution to automate our employee on-boarding paperwork (forms w-4, i-9, etc). I know Zenefits offers a free option, and many start up employers use DocuSign or HelloSign, or their API, to take care of this administrative work electronically. I was wondering if there are other options available that I'm not aware of. Each of these choices gets me ~80% of what I need, but I feel like there must be something better. I am curious what others are doing. To share a little more about our use case: we hire 50-100 employees temporarily during the summer, so the administrative load can get quite high. I'd also, if possible, like to integrate everything with our software so new hires only have to log in to one new system before starting work. Many thanks!