Try NOT to use the Post Office in the little plaza on 535. If you decide you want a post office, hit the one on Turkey Lake Rd. (Palm Parkway turns into Turkey Lake)
For UPS-You have a few choices you can call 1-800-PICKUPS and they will come to you and get the boxes.
Or you could go to ups.com and they will show you locations for UPS stores or Customer Service drop off locations.
The closest UPS store to WDW is located in Summerport. Head over to Downtown Disney area and follow the road directly in front of Saratoga Springs until it T's at CR 535 aka Winter Garden-Vineland Rd. Turn left and follow this road approx 3 miles. It will be curvy and you will see residential areas. Just keep on until you see the signs for Summerport. When you get to the corner of Winter Garden Vineland Rd and Ficquette, go straight there will be a PUBLIX plaza on the left side. There is a UPS store in here.
If you would rather just take a little ride and go to the official UPS drop off customer service center, there is one on Taft Vineland just off Orange Blossom Trail. Go to ups.com and search for this location.