Hi there @mikebrother95.txg,

Welcome to the Community and thanks for posting your question here! Hope I can offer some info and assistance with your question.

1. The first part of this question is interesting because you shouldn’t have to create a custom order value to manually update. When integrating with Shopify, you should see that there is already a pre-made $value property that you can call and have updated on a customer’s order. This documentation looks into what metrics are pulled from each event. 
2. After finding that you can call $value to be used in a Flow (see example below), everything should be settled.  

   

   As a note, for dynamic tags, you can only pull data from the event that is triggering the flow, so if the customer's total order value is in the event data for the current event, you could use it. Otherwise, you would need to add the customer's total order value as a profile property on the profile, which you could then pull into the flow or a campaign. This guide walks through event data and template tags. If you are using the $value, you can typically use the tag:  {{ event|lookup:'$value' }}

I would advise double checking your integration to see if you can call the $value property for your Flows and user profiles as well.

I hope this was helpful and have a great day!
Alex