I bartered on gmailswap.com, sending someone a bicentennial 50¢ US coin in exchange for an invite. The envelope made it to the recipient, but the coin fell out

I bartered on gmailswap.com, sending someone a bicentennial 50¢ US coin in exchange for an invite. The envelope made it to the recipient, but the coin fell out in transit because I was young and had no idea how to mail coinage. They graciously gave me the invite anyway.