One possible approach is to send 2 emails back-to-back, with the first including all but the last 4 numbers of the card, and the second containing the last 4 numbers plus the expiration date. Or, call them up. Fraud is not a huge concern, as you wouldn't typically have to pay unauthorized charges.
1111-1111-1111-xxxx
xxxx-xxxx-xxxx-1111, exp. 11-11