I have an app that is a social commerce platform and we have products that are being reviewed. We want to tap into a retailer who has a Magento store and we want a customer to place that order through us but let the retailer do the fulfilment but without them leaving the journey. What's the safest way we can propose to do this for the retailer? Our app is built in flutter.
Assuming you want the Magento store to handle the entire order process and not just the fulfilment then the easiest way would be to use the REST API (or if the Magento instance is on a more recent version there is also the GraphQL API). I would recommend you start by reading the REST API Reference here https://devdocs.magento.com/guides/v2.4/rest/tutorials/orders/order-intro.html as that has a great tutorial to get you started with. For the payment processing you will still need to implement in-app payment processing so you can pass a token to the
payment-information API but most payment providers have a SDK for native apps. I just did a quick Google and Braintree, Adyen and PayPal all have SDK's for flutter.
Get the retailer to give you access to their staging/test Magento environment so you can get up and running without affecting their live site.