We always ship our packages from NJ to the resort and have them arrive the business day before we are due to arrive. We also ship via UPS and use 3 day with signature confirmation which is guaranteed delivery by end of 3rd business day. We can also track our packages online as well and we even know who signs for our package at the dock.
We used USPS the very first time we shipped and our package didn't arrive until two days after we did. At that time, we did not realize that the USPS doesn't recognize Disney internal street address's and the package's are sent to a central sorting facility on Disney property. They are then forwarded to the proper resort. Now, I am not saying that the package won't be on time via the USPS as a lot of people ship using them but for me, I like to know that my package is delivered on time and signed for. Have a great trip.