Sometimes extra treats can be put into the box from home you can’t get elsewhere to enjoy on your day off from the trail. Especially treats you may not find.
A roll of toilet paper in a Ziploc along with some baby wipes is good. For women, light pads are helpful. If you know approx when you might need feminine supplies, it helps to have that in your drop too along wit any meds you might take.
Medications – The main reason for mail drops. I have a set of personal meds and vitamins I take (see the first aid blog for what I add vitamin-wise. I usually carry enough meds for ten days. Be sure you are ok on the homefront with your prescription meds and plan ahead (you can ask for "vacation refills" ahead of time to pack into maildrops). I have added a sandwich-size Ziploc with some extra Advil and Tylenol
Maps and pages copied from the Thru Hiker Guide or the Companion you need for the section you are hiking.
I've also added for long distance ventures –
Some brand new Ziploc bags to replace the ones I use in my pack
A few extra band aids and some duct tape to replenish the first aid kit
Gear: If you are thru hiking the Appalachian Trail northbound, typically gear is switched out around Pearisburg, VA for the summer (after you pass the Mt Rogers area) or by mid May and pick up colder weather gear at Glencliff, NH for the Whites, including clothing and sleeping bag (if you go with a different bag). NOTE: At Glencliff, NH you can also send your box to the Hiker's Welcome Hostel or earlier to the PO in Hanover, NH. Southbounders - your cold weather drop depends on your start date, but you will need colder weather gear usually by mid October (I had mine for southern VA to Springer by then).
Other Related Blogs
List of AT Maildrop Locations
List of good Grocery Stores on the AT
Mailing blog on Mail Drops (USPS info)
Town Etiquette for Hikers