I've been sending
ebay coins for 18 years. Most of what I send are single coins of lower value so I don't want to charge the buyer for tracking costs. I tried to find the easiest, cheapest way to wrap them and have come up with this.
First, buy a stapler like the one shown. It can easily staple a 2x2 to a piece of cardboard, unlike most small elcheapos. It also has just the right jaw length to use on the next step.

Secondly, buy dollar store envelopes 4 1/2 x 6 1/2 for 1-3 coins, business size for 5-8 coins - cost is 1 or 2 cents each.
Third, cut cardboard into strips slightly narrower than the height of the envelope, and at least two inches longer. Use flat cardboard like cereal boxes, not corrugated.

Fourth, fold the long end over the coin. If it's in a 2x2, just put a staple in the middle (like in the photo) to hold it in place - the stapler just goes past the 2x2 and goes through the top and bottom of the cardboard. Crush the exposed staple end with the top side of the stapler. If it's a loose coin, use 3 staples, one in the middle and one on each side. Crush all staple ends.

Put it in the envelope with the fold towards the back of the envelope. If you are wrapping more than one coin, you can cut a piece twice the length of the envelope, or 2 separate pieces, and staple coins between them (3 2x2's can go in one envelope and stay flat).
Never heard any of my customers having a problem with the packing and I get many feedbacks mentioning the secure packing (you can check - tigert on
ebay).