Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 208; Flight Crew Total 10717
Deviation / Discrepancy - Procedural Hazardous Material Violation; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
General Flight Cancelled / Delayed; General Maintenance Action; General Work Refused
Human Factors
About 10 minutes prior to scheduled pushback from gate; a ramp supervisor entered the flightdeck and introduced himself.  He had a phone with pictures he had taken of cargo he wanted to show me.  It showed a torn box with a propane cylinder in it.  He proceeded to tell me that one of the ramp loaders while loading a shipment of cargo noticed this box was torn open and saw a cylinder inside.  He inspected it and found out it was a propane cylinder that was full of propane.  The cargo was from the USPS and was a load of the same type of boxes weighing 1800lbs.  The rampers called their supervisor and he came to inspect the cargo and took pictures of it. The hazardous material was not declared and their was no record of this type of cargo.  It was the supervisor who came up to the flight deck to notify me of this. He told me the boxes were loaded with full propane cylinders. The gate agents were wanting to close the main cabin door about 5 minutes before push and we told them to hold off because they had to offload the cargo.  The hazardous cargo was offloaded in about 10 minutes and the ramp supervisor told me it was all offloaded and we were good to go. We proceeded to taxi out and the rest of the flight was uneventful.
B-737 Captain reported refusing USPS cargo of undeclared propane tanks that rampers discovered during cargo loading.