Neiman Marcus requires that you have Chanel purchase history from inside NM. Stand-alone Boutique purchases don’t count. I experienced this last year. However, you *can* purchase Chanel shoes from NM without a purchase history.
What you can do is contact a boutique SA (perhaps the one you bought the shoes from?) and have them remote send the item to you.
Alternatively, if you have the ref number for the item, call customer service. They can send your request to a store and an SA reaches out to you. I’ve done this at least 5 times successfully.