I built a checkout API. Using it incorrectly could easily result in a user ordering items they didn't want to, paying an amount they didn't expect, selecting th

I built a checkout API. Using it incorrectly could easily result in a user ordering items they didn't want to, paying an amount they didn't expect, selecting the wrong shipping, billing the wrong card, and so on. Building the API in such a way that it was hard for clients to get this wrong was a very key concern for us. Separately though, I gave a better motivating example in my other reply here. tldr: what you might be paying for may come with stipulations that are not clear in the data – such as how up to date data is when used. Those stipulations may come through many layers of data source, not necessarily from the layer you pay.