There are many different kinds of platsic. Some are "porous" and some are "non-porous." "Porous" means it has tiny molecular holes in it. Certain plastics with holes large enough will allow small amounts of odor through allowing a drug dog to smell it. Do your research (i have not so I can't fill in these holes for you) and be sure your shipper is doing it right. Moreover, don't buy mom from a mag ad or a board post unless you just have the money to burn. I have personally never mailed anything but I know it's possible. I knew one guy about a year back who had meth DHL shipped from cali to the midwest. So I know it is possible but I don't know the details of the preperation. I went out of my way not to be involved with those people as much as possible. However, I did watch the guy take the package from the DHL guy, & open it up to a couple of onions, vaccum sealed it appeared (idk for sure, he dissapeared quickly).
However, if you are unsure about your shipper's methods and their security, I reccomend holding on to the package for a couple of days without opening it. They will, upon disovery, continue the package enroute and then bust the person recieving, and attempt to cut you a deal in exchange for your supplier (who, while obviously not using good enough product security will definately cover his own ass, hence they come for you not the shipper). Cops are pretty impatient when they don't know whats going on and will likly jump the gun within a few hours of recieving the package. However if they do, and you have not opened the package yet, you have plausable deniability. If you "just opened" the package and you see them coming make a casual attempt to call your local police department to make it seem as if you had no involvement and were attempting to report it. You'll likely still be arrested but the phone call may still give you plausable deniability in court if you have a good lawyer.
NOTE: As I stated, just because you recieve the package, DOESNT mean they don't know you have it. Be careful, give it at least a day if not three before opening it. If they jump the gun and bust you with an unopened package two days later, say you are unfamiliar with the sender, were not expecting a package, and what with terrorism in the mail and all you were just scared to open the package, and rightfully so., it seems =).