We used UPS to ship a large tub down. We choose to send it down so it would arrive a week before we did. That way if something went wrong, we'd have enough time to implement a contingency plan.
However, UPS ships to the Business Centre and they charge a fee based on the size of the parcel. I think $15 was the peak, and $10 was what we paid.
The IKON (Business Centre) staff will being it to your room, but if what I was told by them is true, as "operating partners" they do not have keys to the rooms and therefore you must be present to receive your package.
When it came time to ship it back, we called the Business Centre, they came and picked up on a cart and we followed them down there to complete the paperwork and pay the $10 pick up fee.
NOW, if you're doing USPS, it's my understanding it bypasses the Business Centre and goes to Bell Services where there's no fee. Whether they can leave it in the room unattended or you "must be present to win" is a unknown to me.
Shipping it back, you'll have to work through the Business Centre (it's located over in the convention centre so if it's a heavy item you might find it worth the fee for them to cart it over for you.).