Just be fair smoothie. If you can ship a coin for $11 don't charge 4 times the price or 6 or whatever.
It is sort of ironic, these coins represent freedom from repressive systems... or so I thought....
He only wants to ship registered mail so people can't complain they never got it, even if they did. That costs more than $11.
This is used all the time to ship silver coins. It is insured to $400. If it is insured, isn't it essentially the same thing? If it wasn't wouldn't people just say I never got it and collect $$$?

I am shipping priority mail and also the coins come in a box (not an envelope). Try fitting a two coin leatherette case in an envelope.