Maybe I'm just having a tough time figuring out the use case for your free tier, but if I make less than 10 orders/month, exactly why would I go to the trouble of implementing an API?
I'm guessing people that are just tinkering with the API for the fun of automation. For example, I could use this to order new socks every few months. Just because, yay, unexpected socks in the mail.
It might be the case that people are just tinkering with this service for fun, but the "yay, unexpected socks" use case seems like a solved problem with Amazon Subscribe and Save lists (http://www.amazon.com/Subscribe/b/?_encoding=UTF8&camp=1789&...). To each their own, but I just don't see myself reinventing the wheel with this API (not to mention paying an additional 5%) to buy socks/paper towels, when Amazon has already done that leg work for me.
I do, however, agree with meritt's sentiment above...zinc.io could be used to create some valuable services for existing businesses (rewards programs, et al), but as a user of this API, I think it'd be much more cost effective as a fixed per-order fee structure.
