Nick here from Shopify.
Great question. Something to remember when selling on Instagram via Shopify is that the product would need to be created in Shopify first then connected to your Instagram profile to sell. There wouldn't be an app or a way to turn an Instagram post per se into a product on the Shopify store, it would happen the other way around in this case.
Shopify has a helpful guide which goes through all the steps to set up the Instagram Sales channel for your store which you can read here. You can also see all the apps which are connected to Instagram or selling on Instagram within the Shopify App store here.
Hope this helps and answers your question!
All the best, Nick