I used to use coins to show scale on items and thankfully never had anyone say "where's the 5c I was supposed to get"  Now because I sell internationally, I use a combination of holding things in my hand, and showing them next to a ruler with cm and inches on it. 

No one better ask for my hand!!  

I had an interesting one as well last night, MB.

US buyer, purchase was nearly three weeks ago, and messages to say the order hasn't been received (not unusual lots of packages are taking around 3 weeks at the moment). They said they looked at the address and it's incorrect, then gave me the correct address. I sat there for half an hour thinking "what the blimmin' he** can I do with this?" Then I looked at the order details, and the address was identical to the one in their message. 

O_O