Presence and timing of customer_id field in Orders API

I’m seeing customer Id inside the Tenders, inside a transaction. Is that your suggestion? My use case is an OrderCreated/Updated webhook that looks for completed orders. When order is completed, I check to see the items purchased and if one is a particular category, then I generate a special templated email with instructions, pdf, etc… so since it’s in a webhook and time is not a huge concern, my plan is to get the Order via Batch Retrieve; check for customer_id. If it’s not there, get the Transaction by OrderId, and check for customer_id in Tenders array.

Any other thoughts/ideas on how best to confirm identity of a customer for purpose of sending special instructions/email to them based on the items in a particular order?

Thanks for your help!