After I have seen the stealth of about ten different vendors, I'd like to take this opportunity to discuss how it can (easily) be improved. Below are some ideas, but let me start with:
Customers: Do not give 5-star reviews for unimaginative stealth!
As a customer, you are responsible to incentivise vendors to constantly up their game - it may well save you some jail time or a ruined career and/or private life some day! A silver baggie folded into blank paper does not stand up to scrutiny when the envelope is opened (accidentally or by customs) and should be rated as mediocre, because it can easily be improved.
Vendors
CREATE A STORY! Your mailings should tell the story of a legal, common mail transaction and all parts that you can influence should fit that story. So let's go through these variables:
Postage: I'll start with this because you may not have complete freedom here and your story depends on what you (have to) choose.
Basics: Make absolutely sure you put enough postage on your mailings. Make absolutely sure that stamps (if used) will stick to the envelope (fresh, self-adhesive or using the proper amount of moisture)
Discussion: Most mail today is sent in bulk by large businesses (banks etc.) and in a perfect world, that is what you would go with. Unfortunately, these self-franking systems are probably not available anonymously in your country. Maybe you have access to a large mailroom and can channel your mail through that? If not, and you have to resort to stamps, that will limit your story to those that naturally include stamps. Example: credit card statements could be excellent stealth because they're very common. But a credit card statement with a stamp on it dropped in a mailbox somewhere in a small town? That story doesn't make sense. Don't use it.
Given that you're limited to stamps, the next question is: what's a typical legal letter today that uses a stamp? I'd say it is (a) private letters and (b) small businesses and I'd go with (b) for my story because it's probably larger in volume and it makes some of the next steps easier. It's also more natural, because you basically are a small business.
Customer Addresses
Basics: Find out the format your mail carrier recommends. The country, for example, should usually be written in capital letters. Tell your customers the format to use and use an automated method to check it. Make sure the address label (if used) firmly sticks to the envelope. Make sure the full address can always be seen when using windowed envelopes.
Discussion: What's your story again? Make sure the customer address fits in. Private letters are often hand-addressed so if you want to use that story, break out the pen. That's also why the 'private letter' may not be a good idea: You don't want to handwrite addresses (time, possibility of errors, traceable handwriting). So you're printing either directly on the envelope, on address labels or using windowed envelopes. Most businesses would probably use either labels or windowed envelopes. Make sure they match the professionality someone would expect for the story you chose. For example, I once received a letter purporting to be from a medium-sized company with the address printed on standard paper, unevenly cut with scissors and glued to the envelope. TERRIBLE STEALTH! The story doesn't add up and the label could easily come off. When the label comes halfway off, the envelope gets stuck in a machine, ripped open and your customer is fucked.
Return Address
Basics: It should be a real address. It should be somewhere close to the mailbox you're using. It should be formatted correctly. It should usually be printed on the back (check your mailer).
Discussion: This is actually the main part of the story you're telling because it will usually include a business name and that name is the main clue of your story. Make up a good name for your type of business. Make sure someone looking at the letter instantly 'gets' the story, i. e. include the type of business you're pretending to be either in the name or in a slogan or something else.
Envelope
Basics: Buy the highest-quality envelopes you can find that match your story. Make sure the flap securely stays closed
Discussion: I'd say that usually, self-adhesive flaps are preferrable. They reduce the possibility of error when moisturizing manually. You also don't want to lick the flap (DNA). Make sure you use the right paper quality and format for your story. Private letters don't use windowed envelopes. Businesses (usually) don't use those heavy-paper envelopes you'd use for a wedding invite.
The Content
Basics: Always include paper. Print something on the paper. Use enough paper to make it hard to feel the product when holding the letter
Discussion: This will take the most time. You want to create some content that fits your story. Usually that will include at least a form letter with some text that makes sense. Make sure to check spelling & grammar or consider copying the content of some real business. Consider creating or copying extensive content like printed brochures, invites etc. These may allow you to hide the product more effectively.
The Product
I can't say much about this because I don't know enough about the details involved. I'm also under the impression that most vendors get this right - although I can't check for parameters like smell and susceptibility to x-ray etc.
Discussion: This is a bit hard, but maybe there are ways to come up with a story that naturally explains why you're sending out a bag of powder or whatever your product is. Check for legal products that resemble yours and how they're packaged. Possibly individualize the actual product packaging with a product name, parts number, expiration date etc.
Examples
The Wedding Invite
I advised against private mailings above, but one exception might be a wedding invite. It's one of a few types of private mailings that is usually produced in bulk, addressed with a printer etc. Return address: should include two names, one male one female with different last names Customer address: Would usually be printed on a label Envelope: Would usually be high-quality paper with noticeable texture and a deep, V-shaped flap (possibly depending on your location). Should contain some obvious clues that this is a wedding invite (because it's hard to to in the addresses) but those clues should interfere with automatic recognition of postage and addresses (i. e.: on the back only). Content: An invitation printed on high-quality paper. Copy a real example!
The Wedding Photos
A variation of the above, but this time you're sending out one or two photos and a thank you note after the wedding. The photos are useful for hiding the product. Paper stock could be lower quality
The product sample
You're a small reseller in the business of selling powder, let's say it's an industrial coating. Your products should be labeled with numbers, names etc - possibly find something real. You'd use a cover letter thanking the customer for his interest. This also allows you to print the address on that letter and use a windowed envelope. Make sure to include a (possibly photocopied) price list.
The brochure
You're a travel agency specializing in African Safaris. Include a cover letter and a high-quality brochure of, say, a travel package or a specific hotel. The brochure could have a heavy cover with an inside flap for the product. Hey, why not go all the way and create a website for your business. (Maybe you'll even get some customers and turn all legal in the process).
Also
Most printers today encode their serial number in invisible dots on each page. Make sure to buy your printer with cash.
I once recieved some cannabis from a vendor that followed a lot of these principles, I didn't even realise what it was until I had fully opened it. I won't go into too much detail but the package had a return address sticker which was a seemingly legit business. Inside was an invoice for some product related to the business, and the "product" itself which was professingly packaged in a sealed foil bag. There was a serial number and description for the "product" on the foil bag. Once the foil bag was opened there was a hard plastic container with the weed inside in a sealed plastic bag. I can only conclude that the business was indeed legit, and that the owner or an employee were selling drugs on the side.