If sending post I worry more about sorters that bend the envelopes, so you probably want to place the cards in a small packet format, and use a corrugated cardboard sheet both sides of the media, with a inner sheet cut to fit the card, with the 3 sheets taped together or stuck with a water based glue, then placed in the small packet grey envelope and sealed. Millions of these are handled by the post every day with no issue, and minimal damage. Small packet is generally cheaper than small parcel, and also offers an option of usable tracking and insurance as well.
Remember that Dave with his uRulers had to use a Tyvek envelope, with an inner card with them stuck to them, to prevent the rulers from either being destroyed or lost as they tore the envelopes. The rulers bend a lot better than silicon, and do not break in doing so.