Adding more 'Winning Metrics' to A/B test in flows?

I have a flow which is designed to convert people who have received an estimate from us. The main CTA is for them to get in touch to confirm a booking or continue with their purchase, not to finish their purchase online. 

I want to A/B test this email to understand which email is causing more people to convert. The only metric available to me are ‘Open Rate’ or ‘Click Through’. Neither of these works for me as my Open Rate is already ~80% and there is no clickable CTA. Is there no way to add the conversion metric instead? Ideally I want to know which email is ultimately driving more sales through an increase in the key conversion metric, in this case; Sales_receipts. I realise that I can track this in in the report for the test overall but I cant seem to use it as a Winning metric.

Is there a way to configure the A/B test metrics? Thanks