I’m going to put in another plug for maildops. If you have a home support system and especially if you make much of your own food, they can be awesome. Jeff and I maildropped almost all of our food for our two hikes, and had no problems. We almost never ran into problems with PO delays - just modify your plans(speed up or slow down) as you go.
Priority mail is the only service you should use if you go with the USPS, as we did. Free forwarding (by phone), quick delivery (2-3 days from NY to CA). We had no complaints at all, and no lost boxes, either.
Also, on the AT, you can mail cheese in your boxes with no trouble, except in all but the absolute hottest weather. We had an 8 oz. block in every one of out AT drops, and didn’t lose any of them. I don’t advise doing this if your box will sit for a week or more, but if your drops are being shipped relatively close to you ETA, it’ll be fine.
Chipper