Is the post office allowed to check packages

I saw someone post somewhere that the post office is allowed to check your package  .  I wonder what would happen if you receive a cash package from a casino and the post office finds out 

It isn't against the law to receive cash in the mail. the post office just highly recommends that cash do not be sent. Mail gets lost all the time and there is no way to trace cash.

I agree with Blue; I'm not sure whether the post office is allowed to check packages. If you're sanding or receiving something from abroad then customs will probably look into it. Anyway, there are safer ways to transfer money nowadays.

mon·ey laun·der·ing
noun
the concealment of the origins of illegally obtained money, typically by means of transfers involving foreign banks or legitimate businesses.

I was just concerned when i saw that post and then my husband said it could be considered. as such  8'|

Can Postal Inspectors open mail if they feel it may contain something illegal?
First-Class letters and parcels are protected against search and seizure under the Fourth Amendment to the Constitution, and, as such, cannot be opened without a search warrant. If there is probable cause to believe the contents of a First-Class letter or parcel violate federal law, Postal Inspectors can obtain a search warrant to open the mailpiece. Other classes of mail do not contain private correspondence, and therefore may be opened without a warrant.

They are all checking packages. Reason? Border control. Few days ago I wanted to send some heart pills, since hes on vacation in Netherlands. Package is stuck on his way out of country becouse they won't let you take anything like that out of country. And also every package is getting opened, so dont waste ur time or money on that shining papers becouse they gonna get ripped of anyway before they hit final destination.

As far as I know they are not allowed to check the packages.They are only allowed to scan them, but that's all.At the border control too, they are not allowed to open the packages.Only if they scan them and see something suspect, then they are allowed to open.

I'm surprised the post office can look in packages. 

When I mail a package, I am always asked "Are there any liquid flamables, explosives, etc".  I always think, what idiot would say 'yeah, I am mailing a bomb to my cousin who is having problem with a neighbor".
Maybe trust by the Post Office is not in the best interest of anyone
Can you imagine if the airlines asked 'do you have a gun, bomb,  or knife?"  No?  great, you may board the plane.

Postoffice kinda has to check mail, since many illegal stuff may be going via their service. They are doing in our best interest. Never heard of someone missing a part of his mail or stuff like that. If you miss mail with cash in it, people will lose jobs, trust me.

If you send overboard, you package will 100% get opened. I've recived t-shirt and mug from LCB, package was literally eaten and spilled out, but objects inside were untouched.