Where is Breadwood's missing mail?

The mayor of Breadwood has tasked me with finding their missing mail. I've done these steps:

• Talked to the bearded guy in the Breadwood bunkhouse
• Visited the Postal Way Station and got the route information from the cabinets
• Gone back to the Dirtwater Post Office and obtained the cheatsheet for the mail routes from the postal wizard
• Returned to the Postal Way Station cabinets to get the real route information
• Stopped by Chuck's house...

And at Chuck's house the trail goes dead. Literally. Chuck strongly implies that the mail carrier is dead, but where is the mail and how do I get it?

The dungeon door is locked with an unpickable plot lock, and all of the conversation topics inside the blood and breakfast don't go anywhere.

On his spice rack, switch the labels around. Then ask him about tea again, and he'll make some for himself. When he's out, you can sneak into his room and get the key to the dungeon.