Two separate problems get mixed together. First, get the items out of mail before the window closes. Second, hand each cipher to the matching camp NPC the first time, or you lose the one-time bonus and only get gold. The mailed ciphers stay in inventory after claim. The mail itself expires.

1Do This Now (In Order)

2NPC Word Matching List

NPCHint Words
Sigrid
Victory Wine / tavern, Red-Haired Omen
Residual Heat, Sealing Wax, Acid, Malt, Cloves
Merimo Rugs, Caravan, Hay, Cheese, Camel Bells, Salt Lick
Dramm Fire, Brimstone, Furnace, Anvil, Ashes, Smoke, Mineral Vein, Oathbound
Dew Homeward, Anchor, Prophecy, Spindle, Weaver, Echo
Soekja Stone Steps, Tides, Signpost, Supplies, Guidance, Return, Wings, Praise
Pip Mantra, Tree Hollow, Secret Cache, Elder Edict, Plucking, Score, Haggling, Hoarding
Cazarro City State, Intelligence, Insight, Balance, Covenant, Interest, Barter, Blood
For the mailed pair: Ashen Cipher goes to Sigrid. Abyssal Cipher goes to Merimo. Wrong NPC still consumes the cipher and you lose the bonus (often a Ripple Gem or epic materials). Do not take Abyssal to Sigrid just because she is the cipher NPC.

3Decision Points

Time pressure
Claim now vs wait until Level 5
Claim now. Level up later. Mail dies at the timestamp. The items do not.
Wrong NPC
Sigrid vs any NPC
Any of the seven can open a cipher. Only the matching one pays the first-time extra. If you already handed Ashen to the wrong person, the bonus for that type is gone.
Duplicates
Two of the same type
First correct hand-in gets the extra. Later copies: gold only, around 450.
Source
Raid-found vs mail ciphers
Mail ones are already extracted. Raid ones are lost if you die before extract. Do not treat them the same.

Common Mistakes to Avoid

Reference Image

Official announcement image for the Second Wave mail:

Mistfall Hunter Second Wave mail announcement