Ordered 2 Items & Seller Only Sent 1

I ordered 2 of an item on ebay and the seller only sent 1. I let the seller know and asked if they could send the 2nd item and they responded back saying I only ordered 1 when the order clearly shows 2. What are my options to get a refund? I see I can “Return Item”, but there is no way to select that I only received part of an order. Appreciate your help.