Appreciate the feedback. We have realised we are not able to use checkout extension as we are not Plus Developers, so we are looking for different ways to provide our discount logic - possibly Functions can do what we want.

OK - so, we are not Certified Plus Developers  Which I believe means we cannot even develop on a Plus sandbox, let alone publish it.  Seems amazing Shopify partner support didn't look at our account and say "Check extension is for Plus partners only...

Thank you - we are checking and will update you.

Update from their support: "Our development team reviewed all the details you provided regarding the issue you were experiencing with testing your app, however they have determined that the nature of the issue being an issue with deploying a checkout...

Replies from support suggest that it is live and this should be working. 

Did you manage to resolve the issue?

FYI - I am getting suspicious that Shopify have released this at a Developer level, but have not sent it live on their production level - if so, it means they have people like us investing our time and money into coding things which dont actually yet...

We've faced the same problem, filed a report with Shopify Support last week and not heard anything despite chasing them every day this week  We have an app ready to go - the checkout extension works fine in our dev environment but when we deploy to ...