Your best bet would be to use impact-resistant plastic vials or something similar and put those inside a bubble envelope mailer, then write the words “non machinable” on the front of the envelope.
A small parcel package may also be used in place of the bubble mailer, make sure all seams are well taped with quality tape for extra security.
Some of the small parcel sorting machines don’t play nice with some of the small parcels… many times, taping seams is necessary to keep the contents inside the parcel during rough handling and/or transportation.
Non-machinable means it will be sorted by hand and won’t run the risk of being put through any letter automation machines. The automation machines will definitely crush and destroy anything that doesn’t lay flat in the envelope, anything that can’t bend or anything that is thicker than 1/4”. The wheels and rollers in these machines can force loose coins to rip through their envelopes and become lost in the machine, soft bulky items will get destroyed right away.
It will cost a little more for postage if it’s non-machinable, but it won’t get destroyed before it gets to its destination either.
And yes, I work for the USPS.