I've seen just about everything when it comes to shipping notes by mail. Everything from notes put in basic envelopes with nothing to protect them, to packaging so thick and sturdy it could survive being run over by the postal truck.
You don't need to write instructions, like "Do Not Bend", on the outside of the package if the package is rigid. By my experience, letter carriers don't make a habit of folding letters. Anything you put on the outside of your package could tell the person handling it that there's something of value inside, increasing the likelihood of the package never reaching its destination.
You should always use security envelopes instead of plain white envelopes. They cost slightly more, but the printing inside the envelope is very suitable for disguising what's inside.
Canada Post has a variety of options when it comes to lettermail. Everyone is familiar with a basic letter. A standard size envelope weighing less than 30 grams is currently 63 cents. This type of packaging is appropriate for circulated notes which do not need something like cardboard for reinforcement.
The type of package I recommend as the standard format for sending notes is the oversize lettermail. Check the canadapost.ca website for details of how long and wide the envelope can be. The key to this format is that the letter can be up to 2.5 centimetres thick. This allows you to use corrugated cardboard to brace the notes inside the envelope, even if you use a bubble mailer. Needless to say, you should put notes in plastic sleeves before sealing them inside pieces of cardboard. Thinner cardboard from food packaging, like cereal boxes, is indispensable and can be used in conjunction with thicker cardboard. I recommend one piece of corrugated on one side for rigidity, and one or two pieces of thinner cardboard on the other side helps keep the thickness of the package down. An oversize letter runs $1.34 currently, and the post office sells stamps specifically for this type of package.
Oversize letters are safe to deposit in mail boxes and travel with the rest of the mail.
There are all kinds of plastic currency sleeves. Most will hold the note(s) in place, but some will not. I strongly recommend that you do NOT put tape directly over the open end of the sleeve. Notes inside the sleeve could slide up and come in contact with the tape, leaving a residue on the notes. Instead, here is a better idea:
- Cut a small rectangular piece of white paper (a few cms long by maybe 1 cm wide);
- Fold the paper lengthwise in the middle;
- Put the folded paper over the top of the plastic sleeve's open end.
- Take a piece of tape (scotch tape, or whatever) several cms long, fold over a little bit on each end so there's a non-stick "tab", and put the tape over the folded paper orthogonally so the ends stick to the sides of the sleeve.
The recipient can easily remove the tape and paper and be assured that no damage was done to the notes in transit.
The type of note and its value will determine what type of packaging is most suitable. Just use "common sense". Of course, if the person buying notes from you does not want to pay for shipping, feel free to adjust your habits accordingly.
But most people don't raise a stink over being charged $2 for an oversize letter.
It's Canada Post's job to deliver the mail. They should not receive kudos for doing their well-paid jobs.