Within a parcel box, there is a chance to acquire the following rewards for your efforts:
A tradeable outfit style page for the new High Rock Spellsword armor
Crafting materials, provisioning ingredients, alchemy reagents, runestones, or a motif style item
Small chance of a gold-tier booster or a research time scroll
Furnishing or provisioning recipes
Repair kits, siege repair kits, or a small chance for an Impresario group repair kit
Treasure Maps, survey reports, Imperial City Vault Keys, companion gear, or a Random Currency Pouch
After doing my first ~500 boxes:
Style Pages - These seem to be capped at 1 per day per account, like "normal" event style pages.
Survey reports - These do not seem to drop.
"gold-tier booster" - Have seen 0. (~40 research time scrolls though).
Motif Style items - These do not seem to drop