Americans may be using the post office less these days, but one kind of delivery is seeing a surprising surge: Packages containing drugs, amNewYork has learned.

The U.S. Postal Inspection Service reports a spike in such deliveries – predominantly of marijuana and cocaine – in the U.S. as well as the Big Apple, up a whopping 371 percent nationwide since 2006.

Inspectors gave an exclusive tour of the city’s midtown facility on Ninth Avenue, where the drugs are kept as evidence. Crooks are concealing the contraband in creative ways, deploying picture frames, Tupperware, even dog food.

“They think the post office is a cheap and efficient way to send it,” Assistant Inspector-in-Charge Thomas Boyle said as inspectors showed off successfully seized parcels, which must eventually be destroyed.

In fiscal year 2010, inspectors made 285 seizures in New York, including more than 2,800 pounds of marijuana and 110 kilos of cocaine, resulting in 75 arrests.

Nationwide, inspectors found 37,700 pounds of illegal drugs in the same period, up from just 8,000 pounds in fiscal year 2006.

Precisely what’s powering the surge remains unclear, although Sen. Charles Schumer (D-N.Y.) this month called for an investigation into an online “Silk Road” network that allows people to buy drugs and have them mailed to their homes. Mailed drug packages typically originate in California, Phoenix and Puerto Rico, Boyle said.

Criminals consider mailing drugs an attractive option because mail “is protected by the Constitution. With your stamp, you’re guaranteed privacy,” Boyle said.

Inspectors can’t open a package without a search warrant, except when it appears to pose an immediate danger – such as a bomb threat.

Postal employees are trained to profile a package and its sender, looking for signs that might indicate something isn’t right. They’ll also use X-ray machinery to help determine what’s inside. While that doesn’t require a search warrant, the machines are typically only used if there’s a perceived threat.

Drug enforcement, however, isn’t the unit’s usual beat. The inspectors normally end up cracking money-laundering schemes along with the NYPD and FBI. While the city’s 90 inspectors do the detective work, there are also about 200 postal police officers on the front lines.

“We’re pretty much a well-kept secret,” said Lt. St. Clair Maynard, who wears a badge and carries a gun and handcuffs.

The postal facility even has a couple of jail cells where suspects arrested for committing a crime on postal property may be held before seeing a federal magistrate.

“The thing about New York is you never know what’s going to happen,” Boyle said.

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Return to sender

Some things shouldn’t be sent through snail mail. Besides drugs, however, people have packed some questionable – and downright illegal – items:

•A Minneapolis woman was charged with animal cruelty after trying to mail a live puppy in a box in January. Workers only realized the pooch was in there after the box moved off the counter. Not all animals are off limits: The Postal Service allows shipping of day-old poultry, adult fowl and cold-blooded animals, such as lizards and bees.

•In 2009, a South Carolina post office was shut down after a bomb-smelling odor was detected. Firefighters determined someone had mailed a dead skunk to a local taxidermist.

•Postal employees in Richmond, Calif., found three grenades while moving packages in 2007. Authorities found a total of eight grenades, and while they turned out to be hollow, they are also illegal to possess.

•A German man was sentenced to six months in jail in May for illegally importing tarantulas into the U.S. He took in $300,000 as part of his worldwide mail-order business.