I used UPS Ground, and it was perfect. If you go to their web site, you can see exactly what day it will arrive by entering info and then decide when to send it. They only deliver on business days, so if you will be arriving on Sat. or Sun., have it delivered on Friday. I put some info on it (like my name and my resort arrival date), and the resort let me know that I had a package when I check in. And if you know the weight, you can see how much it costs.
Do not us USPS - from what I understand, USPS mail goes to a central location and then is distributed to the resorts. With UPS Ground, the package is delivered to the resort. And UPS Ground gives you the exact date of arrival.
I agree that I wouldn't send something heavy like drinks or anything perishable. But it was great for diapers, wipes, disposable sippy cups, coffee, little cereal boxes, non-perishable snacks, etc. It was much easier than I thought. And it was less than $10 to ship (I think that it was $7 and some change).