I've done this twice at AKL and twice at the Swan/Dolphin. I've never been charged; once it took them a little while to locate my box. I used this to send little snack packs of cereals and little goodies for my kids - candies (not chocolate), glow sticks, our over the door shoe holder, floaties, etc. This is so I only bring carry ons on the plane, and our grocery stop is for milk, water, and some other odds and ends.
1) I make sure to send it with a delivery confirmation, so I know when the resort got it
2) I make sure to let the resort know it's coming (call the front desk)
3) Put the correct resort address on it, as well as your name, your confirmation number, and check in date
It was always a fairly small box, usually cost about $25 to ship. I wouldn't send anything too big or heavy. Sounds like yours won't be. I'd do it if you are doing so to avoid checking luggage, which is a hassle I prefer to avoid (of course for our upcoming cruise I don't think that's possible!)
Good luck!