- Most small/medium sized ecommerce stores we were targeting with our product were using Shopify
- If they are using Shopify, they're used to the easy installation of an app... And if they're not (by nature of being small / medium sized) they are likely to be dev-averse or slow at implementing a custom API.
- The Shopify app developer ecosystem is — on the whole[0] — a rich resource and easy to get started. If you know React, they have great tutorials to get up and running, and also a stellar UI kit (called Polaris) to make Shopify-esque interfaces pretty easily.
Because Shopify apps (even ones embedded within the admin UI - which we decided to do) are basically websites, you can leverage your own agnostic back-end and go for a best-of-both-worlds approach.
[0] - The downside to integrating into Shopify APIs is there are definitely some (surprising) gotchas, and the documentation does have some missing pieces due to churn (they release new API versions every few months). The dev forums and Git repos are, generally speaking, pretty active and most questions have already been asked.