Post Office
Items in your Storage Chest count toward completing orders, so you don't have to carry the items in your inventory! ONLY completed orders are reset every 24 hours. Each character has their own set of Box upgrades. So yea, when you complete an order, all characters get Box Points to spend on upgrades. You'll get extra Box Points at delivery streaks of 5, 10, 15, etc. You can use a Silver Pen to skip an incomplete or complete order without resetting the streak.
Calculator
Steven Bates has made a Post Office calculator in Google spreadsheets using datamined information provided by Xores. This spreadsheet allows you to input the amount of materials you have and what streak you're at to see how much materials of any kind you need to complete an order at the current streak level.
Orders
Starting at 5-streak, you will earn a bonus for every streak in multiples of 5.
Upgrades
Spend your delivery boxes, earned from filling orders, on different upgrades! Each one boosts a stat, and will also give additional bonuses after being upgraded enough times!
The second bonus is gained after upgrading a shipment 25 times, while the third bonus is unlocked after upgrading a shipment 100 times.