I am trying to create a website and mobile app that integrate with Shopify. I am trying to keep the experience as seamless as possible for our users so that they do the browsing of items and cart management on our website (or in app) and then get taken to the Shopify payment system in browser to pay.
The problem I am having is linking accounts on our website/app with accounts on Shopify. I am aware that I could allow the users to enter a username and password in order to get an access token from the Storefront API but as these credentials wouldn't necessarily be the same as their credentials for our system this could get very confusing for the user.
I am also aware of the Multipass system but as this requires Shopify Plus at $2000 which is prohibitively expensive, especially as we have no other need for Plus.
I also know that making multiple purchases using the same email address will link the purchases to the same customer. However as they are able to enter their email address during the checkout process this could easily go wrong.