The delivery arrived Wednesday via UPS to the home on the first block of Heather Road in Upper Darby.
The couple didn't recognize the name on the package and thought it was delivered by mistake.
Not yet knowing what was inside, they left it on the front porch thinking it would be retrieved.
When it wasn't, police say they brought it inside, opened it up and discovered the five pound brick of pot.
Police say it had a return address of Tolleson, Arizona and was mailed from Phoenix. The return address, police say, was fake.
The drugs were wrapped in a styrofoam lining to prevent detection by dogs, police said.
The couple who found the pot declined to talk to the media, fearing retaliation by drug dealers.
Upper Darby Police Superintendent Michael Chitwood says the department sees about a half-dozen similar deliveries a year and can sometimes track down the sender. Previous shipments have included cocaine and heroin.
Police say this appears to have been a "trial run" by some sophisticated drug traffickers. They were likely testing how sharp law enforcement and high volume mail outfits are, as they tracked this shipment by computer for delivery date and time.
"Five pounds? These guys probably have thousands of pounds," said Chitwood.
Chitwood went on to explain that someone was likely scouting the neighborhood, waiting for the package.
"They'll see the delivery truck, they'll go up on the porch, they'll sign, and they'll walk away."
Federal agencies have joined the investigation into the source of this marijuana.