We're in the process of setting up our Google Shopping API feed with Shopify. We're using Shopify's own Google Shopping app and have worked through the majority of all data, inputting into correct fields etc and that seems to have been approved Google side in Google Merchant Center.
We're currently stuck on importing 'Promotion ID' data into Google from Shopify. Although not defined as a 'required' field, we have found this helpful in the past as we can define specific promotions against individual SKU's rather than running one promotion on Google against ALL products, which isn't always what we want.
Promotion ID doesn't appear as a field in the current Shopify product import CSV, so how would be go about importing this Promotion ID data into Google Merchant Center? Can we add it through a custom attribute on the CSV and, if so, how would we do that?