The UK's distance selling regulations are very simple. If the vendor has made an error, they are responsible for all the costs arising in making a correction. If this is the situation, claiming a dispute with DHL is irrelevant to Olivia's duty toward you their customer. They should create the credit for you immediately, and carry on squabbling with DHL in their own time.
By happy chance another Australian just gave a report of the way it should work:
Hit Olivias with this stick: your competitor performs in accordance with the distance selling regulations, what's your problem?