I ordered water from costco.com in the business section and it was a good price - I think around $7 a case total.
I ordered about a week prior and shipped it to POP. When we checked in, I just asked them and they had it ready for us.
The below is from the Pop Century FAQ on the resorts site:
Ship packages to/from Pop Century Resort and home
use the street address noted at the top of this post
for packages arriving to the resort, on the labels first line, include your name preceded by the word Guest and date of arrival (like "Guest Kimberly Smith 04.25.09")
its up to you if you want to also include your reservation number, but name and arrival date are essential
your package will be held up to 2 weeks prior to your arrival
pick up your package from home or ship to home from the Concierge Desk
Pop Century Resorts, Classic Years
1050 Century Drive
Lake Buena Vista, FL 32830