Hi Amy,
Thank you for your question! I indeed ship a box prior to my vacation, so I have an extra hand I'll have free on the day of travel which is a big deal when you have children!
I always use UPS ground, in my experience it has been more cost effective over the US Postal service, and I appreciate that I can track my package on a daily basis. I will do a quick "calculate time and cost" on their website, and I usually ship one day earlier than necessary to create a buffer just in case any internal processing is necessary on the WDW end.
Even though UPS will have a label for the package, I always create on of my own to be sure that it is routed correctly once at the resort. For example:
Disney's Coronado Springs Resort
HOLD FOR GUEST: Karen P - ARRIVAL DATE: MM/DD/YYYY
1000 West Buena Vista Blvd.
Lake Buena Vista, FL 32830
Be sure the name on the package matches the name on the reservation.
I've heard good things about
www.gardengrocer.com, but have never personally used them. Keep in mind that if you are shipping a bunch of your dry goods/snacks, you could just stop by the resort shop, most of which have a limited selection of groceries (drinks, dairy, fruit, bread etc).
Hope this helps!