I ship UPS & they do a 3 day delivery, that is business day. So Sat & Sun do not count towards normal delivery days & they are extra.
So if you are arriving on Sunday, have your package arrive for Friday or if you can Monday, although it may not get there till the afternoon.
I have called and asked them (and will call again in a few weeks) when it should arrive by & they usually say no more then a few days (like 1-3 days). So depending on which day you arrive on ship accordingly (does that make sense?).
My last package to Disney arrived on a Tuesday (we arrived on a Wednesday). It was about 30 or so pounds & cost $15 via UPS. They give you the tracking number so I was able to check it the day before & was thrilled when I saw Lake Buena Vista on the web site (geek I know). We either shipped it Friday am or Thursday (I can't remember).
Just have the resort name & your arrival day & reservation # on the package (I think that is all they want on it).
I love the ease of shipping because if you have a grocery stop, you don't have to rush in an unfamiliar store for 30 minutes & if you do a delivery you don't have to worry that they will get it wrong or they won't have it which would stink if your kid is particular or has food allergies like mine does.
Don't forget to ship some breakfast stuff too. It makes it easier to eat in the room & quicker too, oh & cheaper.