I've done lots of shipping in both first class envelopes, and packages.
The trick with envelopes is the tokens rigidity make your mail "non-machineable" - if they run through the automated machines at the post office they are likely to get shredded.
Here is what I've found works very reliably:
1. Put tokens in a magic card sleeve and tape them shut.
2. Tape the card sleeve to something else, like an folded up sheet of paper or index card so they don't slide around in the envelope.
3. Ideally, get some square envelopes - these are inherently non-machineable, so are more likely to be hand sorted at the mail center (if you use a standard envelope even if you pay the non-machineable fee and stamp your letters as non-machineable, sometimes they get put through the machine anyway).
4. Apply clear packing tape along the outside edges of the envelope to give it more integrity. Do not tape over the postage stamp or barcode.
If you do all that you should be fine, and your shipping cost is:
Envelope + Tape + Sleeve + insert (sheet of paper, etc.) + postage + non-machineable surcharge.
The non-machineable surcharge is 21 cents.
Tokens weigh around 1/2 an ounce each. First class mail can be up to 3 ounces. Standard stamps cover up to 1 ounce.
So - if you are mailing a single token a standard stamp + 21 cents will cover it. If you are mailing 2-5 tokens you'll need to pay more in postage.
If you ship in a "PWE" (plain white envelope) without taking these sorts of precautions I can virtually guarantee your stuff will get destroyed or lost and not delivered.
Having done this for a while, I eventually got tired of the bother and now just ship everything in a ziploc bag placed into a bubble mailer for packages under a pound or so, which can be bought affordably in bulk, with postage around $2.60 which includes tracking.
Here are some nice envelopes that are inherently non-machineable, so should get hand sorted more often than not:
smile.amazon.com/gp/product/B007BFKRUU/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
If you decide to just go with bubble mailers, here are ones that I have never lost a token with:
smile.amazon.com/gp/product/B005R4V46S/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
If you ship bubble mailers, you'll want to be able to print labels, these are cheap and work fine:
smile.amazon.com/gp/product/B001PF1BSE/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1