- Don't ship candy, cookies or other junk food. That stuff is everywhere.
- Any place that has people will have food of some sort available.
- Send box to White Pass to the Kracker Barrel
- Send box to Snoqualmie Pass to the Post Office with a note to keep it behind the counter at the Chevron
- Send a box to Skykomish Post Office
- Send a box to Stehekin
1) Picked up box at the Kracker Barrel
The Kracker Barrel is a minimarket at a gas station. They actually had a decent supply of food. You could almost shop here. What you could do is consider the food you can't live without and send only that. For me that was hot dinners. Then buy junk food for all the rest. I learned in Washington that you really can live on junk food.
2) Picked up box at the Chevron in Snoqualmie Pass
The Chevron has a huge mini-market. The post office is inside the mini-market, that's why they'll put the packages behind the counter. Actually, they put them in one of the beer coolers.
You could resupply here easily. There was all kinds of stuff in the mini-market, even shoes, clothes and books. There was also a grocery store, but I never looked inside. You could definitely skip sending a box.
3) Picked up box at Skykomish Post Office
This is a stop I kind of have mixed feelings about. But you kind of have to stop here. If there is a closer post office than Skykomish and you don't feel you have the need for a zero day, I would suggest you send your box there instead of going through the rigamarole of Skykomish. Skykomish is to the west of Steven's Pass where the trail emerges. Some people go east instead to another town that I forget the name of.
If you go to Skykomish, the host family actually drives you to another town further west called Baring. There's a post office there. I'm not sure why you wouldn't just send your stuff to that post office instead. Anyway, with all the driving around and having to wait for rides between the two towns of Skykomish and Baring, it almost isn't worth it.
Both towns of Skykomish and Baring have very small stores. You could probably make do if you are not picky, but sending a box of good meals is probably a good idea.
4) Picked up box at Stehekin Post Office
Some people try to resupply with just stuff from the bakery. Baked goods are heavy and are not as good the day after they are made. Day old pastries taste day old even on the trail. They have a restaurant and store. The store had a small selection but probably not enough to adequately resupply. You could do it if you had to, though.
After that, it's only about 4 days to the Canadian border and you are finished.