Email isn't private unless you use encryption/PGP /etc., but honestly you should be safe enough using PM here for the transfer. The other option could be to take a photo of the card's PIN and share it privately on imgur, for example - only the recipient gets the share via email. Or, upload a text file on mega.co.nz and PM the url and email the encryption key, so you'd need both to access it. More than one way to skin this cat!