There are four locations where food caches can either be dropped-off or mailed:
Longmire, Mowich Lake, Sunrise, and White River
Note that White River is just a few miles from Sunrise. I recommend leaving yourself two food caches, and if you’re starting from Longmire, your food caches should be sent to Mowich Lake and Sunrise/White River. The NPS recommends mailing your food cache at least two weeks in advance.
Mount Rainier National Park has comprehensive set of instructions and policies regarding food caches and you should reference their site to get the most up-to-date information. Go here to see the National Park’s instructions for food caching.
The NPS also states that there is no guarantee that rangers will be present when you arrive to pickup/drop off your cache, but it’s likely you won’t have to wait long during normal business hours, before Labor Day.
If you’ve found that you’ve brought too much food or fuel, the ranger stations are usually happy to relieve you of excess. In addition, they’ll also take your hard-plastic shipping containers.
Fuel cannot be mailed to any of these locations and must be delivered in person. Don’t stress about this – if you’re only cooking for two, carrying ten days of fuel really doesn’t add that much weight. My partner and I used about 16 liquid ounces of white gas on the trip, and given white gas’s density of 0.75, this works out to a starting weight of 12 ounces. Personally, I don’t recommend hours of additional driving to shave eight or so ounces off of your initial pack weight. Fuel is available for purchase at the Longmire general store.
Your food caches must be packaged in hard plastic containers that are rodent-proof. The NPS recommends a five gallon utility bucket or a Rubbermaid-type container. My local dollar store usually sells containers that are the perfect size for this.