Search around on YouTube. There's a video tutorial from this chick in Canada who explains how to use two pieces of cardboard and one of those food sealing deals to create a business-sized smell-proof envelope that holds as much as an ounce. That looked pretty ideal to me, because every single day, there are millions, maybe even *billions*, of business-sized envelopes posted around this country. Since it's not a package of any kind, that probably automatically saves you at least a fair bit of potential scrutiny. And business folks don't like their shit getting delayed.
Obviously, use a fake address for the return sender info. For the receiver's info, change the name enough so that it'll get there, but different enough that you provide them a bit of plausible deniability. Like Howard Johnson to Howard Johnston or whatever.