Shipping should only be two or three dollars. Or you could just sent it in an envelope. However if you were to do free shipping (now or in the future) you would get much more competitive bidding.
I ship small jewelry pieces all the time. I wouldn't recommend shipping in a plain envelope. It will likely be squeezed out by a machine or get smash/damaged. Then winner will file dispute and request credits back.
Put in a small cardboard jewelry box and than put in the brown/manila large envelope. I get from Dollar Tree store/99 cents store. 6/8 envelopes for $1.00. Send First Class Mail with Tracking about $3.00.
I started with taking my items to the post office. Now I ship online. I have more items to ship and it's cheaper online and I think it's worth it. Usually, online shipping from 0.91 to 2.93 light weight jewelry/1 piece jewelry.