I used them 5-6 years ago but now find the bubble mailers to be cheaper and provide more protection. Small 000 or 00 envelopes can be bought for less than the 18c you mention. They are also lighter.

One of the problems with the sticky corrugated material is that they ship in a regular envelope and sometimes the USPS puts these through the canceller even if you mark them HAND CANCEL and pay the 49c postage for a nonmachinable item. I never had a chip or token damaged this way, but several times the envelope was mangled and they had to complete the shipment inside a plastic bag with the usual apology from the Postal Service.