Several references to checks bouncing bring up the topic of funds transfer. You can use anything that both of you agree upon. It is generally not a good idea to send cash in the mail. However, it is done. If you do it. be sure it's not visible from the outside of the envelope.
Checks are better, and certified funds are great.
Don't forget you can get money orders for very little. They are a great way to pay to reduce risk for seller as well. If you have other suggestions for this page.