I am trying to create a custom order export csv file from Shopify. I want customer id to be included in shopify-exported-csv-file for order.
You sure can! I do it in my App. I add two missing columns from the order export CSV Shopify produces. I add in the cost price of the item sold, and the product type. Strange things missing from Shopify data in the first place, but hey, with a teeny tiny bit of scripting code, you can add whatever you want to that data.
So I upload the Shopify data to my server, using the data in the CSV I decide what to add, and then I rewrite the data and voila! Beauty. I save it on Amazon S3 and include a link for downloading the data.