You both bring up good points, and I had the same questions/concerns when I first considered building this plugin. I reached out to info@archive.org numerous times with this exact question, however I never received a response. I also searched as much of their documentation as I could find for any mention of throttling and/or blacklisting based on save frequency, however I found no mention of anything along these lines. Like all services, The Wayback Machine is not meant to be abused, and I highly discourage users from setting up any functionality that would cause overly frequent pinging of TWM. That said, if you look at the sheer volume of data being cached by TWM continuously, around the clock, in a highly automated fashion, the normal frequency of saving posts on even a high traffic site is significantly less (e.g. reddit.com was cached 387 yesterday). This isn't to say that the questions you raise aren't valid - they completely are, and I wish I'd received a response from TWM clarifying this exact issue. That said, TWM's visual form interface is in reality their public API. Whether you submit your site via their actual form front-end, or by posting to their form's action endpoint, the service on their end knows no difference.

With all this said, if you know any more about these questions than I was able to uncover, by all means I would love to know. Thanks!