Yes, you can ship UPS from your resort. UPS-FedEx arrive at the resorts probably every day. I had a new
scooter but the chair was wrong. Too late to do anything about it until I arrived at my WDW resort. There, I received the new chair by UPS, and at the same time, used that box to send back the old chair. It was all handled by the CMs outside in the guest arrival area and I tipped them for their help securing the box for re-shipment and taking it to luggage/shipping. In your case not sure where you would bring your box to. I would start by asking the Concierge. They might take your box, or maybe have Bell Services take it to the luggage/shipping area (I suggest a tip in that case, same as a piece of luggage).