I always ship boxes down for our trip and can offer my suggestions. Number one is that FedEx ground is by far the cheapest and most reliable shipping service we've used. One time I shipped a 50+lb box and a 35lb box and both together were around $52 to ship (4-5 business days).
Secondly, try to get the package(s) to arrive as close to your arrival date as possible. Fedex gives you the exact date and time (generally) you can expect your package to arrive.
When addressing, make sure you have the exact resort address (know this sounds like basic information but there are SO many addresses for resorts and other areas, I always double check).
Under the name of the resort, put your name-I address mine like this
Disney's Old Key West Resort
Attn: Walt Disney-guest
1111 WDW Drive
LBV, FL 22222
Then on top and bottom of box, I hand write in bold, black Sharpie
"Hold for guest arrival on x/xx/xx"
Hope that helps